1) Because the Internet is nearly everywhere, it means the spying it makes possible has spread to match its footprint. 30 years ago, "on the internet" really was novel, because the public Internet simply wasn't. There were a few big military and academic sites around the world, and the concepts that make today's internet work were already embodied in running systems, but there was little reason for individuals to care about privacy invasion, or having their systems crippled by government malware, because their systems and their privacy weren't at issue. There wasn't a World Wide Web as a portal to nearly every resource online, no "Cloud," and no Blue Coat. Now, not only can individuals get on the internet, but the meaning of that phrase has moved, fast, over the last decade: now, getting on the internet is just a fact of modern life, a banal, automated background fact of the way we stay in touch with friends, deal with bills, find entertainment, get directions, and work. Online surveillance of all the signals we emit and receive (over home internet links, over cellular networks, on landline telephones, even on postcards) might be minimized and waved away as the collection of "mere" metadata, but in reality, if you're reading these words online, and even if you're doing your best to read them anonymously, it means you've almost certainly got a collection of data about you online already.
2) Because "online surveillance" is a slippery slope, and it will only get slipperier. Remember the Clipper chip's hardware-based encryption escrow scheme? Who and how often you email, chat with online, or call on the phone is the tip of the iceberg. Robert Bork didn't like having his video watching habits spied on, and that was before Netflix and competitors made the sorting and stacking of movie-watching habits not only possible but an never-ending exercise in deep data analysis. Maybe you don't care in particular about what the NSA, FBI, or anyone else thinks of your taste in entertainment, but you might prefer them to stay out not only of the information revealed by your current online activity, but also out of whatever things are revealed by future developments. Right now, a relatively small part of the online population uses crypto-currency like Bitcoin; a decade from now, it seems likely to be even more widespread than Netflix is today. Do you want your transactions to be public record, or even public-servant record? Beyond that, the era of ubiquitous, automated surveillance doesn't need you to mail an angry letter, or declare allegiance to an unpopular cause online: Just walking around means sooner rather than later you're likely to be captured on camera.
Access to your medical records almost certainly will be online, too, even more than it already is. Online and offline lives will only get blurrier: Your GPS (and increasingly, that means your phone, too) knows where you've been, and your should-be-private Google Maps page knows where you might have considered going. (Couple that with the cavalier attitude that dominates rules about data that you carry in your phone, laptop or USB data sticks, if you cross, or even come near, the U.S. border.) Think about the meta-data (or what the government might characterize that way) that your reading and viewing habits, your prescription medicine needs, your airline tickets, and your Amazon wishlist could reveal, and whether you'd want everyone's digital dossier to be up for ad-hoc scrutiny in 10 years any more than it already is. You don't want the equivalent of the TSA viewing rooms (for your own good, of course) attached to every stream of online communication.
3) Because you're paying for it. How much you're paying is hard to say, because of black budgets, overlapping programs, and the sheer number of systems that are or could be used to make widespread surveillance the new normal, but the mystery price tag starts out high. If you're an American, or an EU citizen, at least you can be grateful that you're likely only being spied on, rather than actively harmed in other ways; in other countries, the outcome can be far grimmer. How much do you want to pay to build an infrastructure for constantly surveilling yourself, your friends, and your family? Especially one that fails so miserably at even its stated aims?
THREE WAYS TO FIGHT IT:
The good news is, while you can't stop the entire octopus, you're not required to be a full-time victim of online surveillance or the offline surveillance that it seems to normalize. Instead, you can take some simple steps that at least fog the glass a bit. Readers will no doubt suggest better technologies and practices, but here's a short list to start with:
1) Encryption, more often and in more contexts. Encrypted hard drives are now easy to buy off the shelf, or to implement with software per-user. Use encryption when it makes sense, for documents, emails, file systems, or browsing; the more you do, the more normal this becomes — if it's perfectly normal to carry data encrypted, no matter how innocuous, it's hard for merely possessing encrypted data to be vilified. TrueCrypt might not be impregnable, but neither are the opaque envelopes you might put in a physical mailbox: making it harder to spy on you even in small ways beats indifference. Good news: not every layer of security takes much effort for you to take advantage of: Mozilla's move to HTTPS Everywhere is an example, as is the option that many OSes are embracing to offer the user full-disk or per-directory encryption.
2) Avoid standing in front of the biggest targets. If you don't yet, use an operating system like Linux or one of the modern BSDs, at least part of the time. The SCADA vulnerabilities exploited to cripple a key part of Iran's nuclear program exploited a well-known hole in a widespread operating system, and the same can be said of many attacks blandly characterized as "Advanced Persistent Threats." Even a cheap, adjunct laptop running an up-to-date Linux or OpenBSD could make you safer for some tasks online; cheaper yet, you can run an entire Linux system from a USB drive, and yank it when you're through. That doesn't stop a mid-stream listener (which is a very hard problem), but a compartmentalized system like that means you can do your online banking or anything else and be less vulnerable to common malware. (Besides, it's fun!)
3) Tell companies, politicians (for instance, by voting for or against), and the people around you, that you object to being spied on. You can't prevent malicious individuals, governments, (or Google, or Yelp, or your Facebook friends) from looking at some of the data that you emit; you might feel perfectly satisfied with lots of the transactions you take part in freely. But you can minimize the worst consequences by being mindful of what you do or don't mind putting out there, and spreading the word when you find abuses of trust that compromise your privacy.
Online spying didn't pop into existence with Edward Snowden's revelations about mass data gathering by the NSA on U.S. citizens. For Americans, having our communications tapped by government agents (even if by a government that has remained far more benign than have many others) extends as long as the history of the country; likewise for Europeans and others all over the world. It's much easier, now, though, for those agents to put an ear to your wall or an eye on your correspondence than it's ever been before. For those in many countries, taking practical steps to reduce your exposure is a sensible move for more than just aesthetic or philosophical reasons, though, and luckily the range of options for preserving privacy and private communications have advanced right along with the growth of the technologies that threaten them.