Because that's the question I hear being begged when the "you have nothing to hide" counterpoint is brought up. You've got nothing to hide, so let me just look anyway. Sure it's none of my business, and I have no actionable legal right to it, but it's not illegal so why don't you want me to see it?
The reason why we don't give broad general search powers to the government (or at least, in theory don't give the government broad general search powers) is because the government can prosecute people, and given enough information they can find a reason to prosecute anyone. Which then fully opens the floodgates to selective prosecution: anyone who's disliked by someone in power will simply be prosecuted and likely convicted and jailed, because everyone will have done something that an all-seeing government can prosecute for.
Cardinal Richelieu is alleged to have said that, given seven lines written by the most honest of men, he could find something in them to create a capital offense and have the man hanged. I'd rather not live in a society where anyone has that kind of power.
What the NSA is doing right now, and what the FBI would like access to, is in no way similar to taking pictures of you on a busy street.
What the NSA has is every intersection wired up with cameras, recording 24 hours a day, and all of it indexed with facial recognition and license plate readers.
No organization should have that much information or reach at their fingertips, no matter how virtuous the mission or the people working there.
Is it? If i'm having a 1 to 1 conversation with someone online then that is a private conversation. There monitoring my traffic is more like opening the letters I would be sending that person.
More fundamentally though, the meat space public/private divide does not map at all to the cyber space one, all analogies and laws to apply one to the other are flawed.
Oh please. They can probably harvest the lot. It'll be some algorithm that deems you worthy or otherwise or gets you on an "of interest" list. Let's keep "personal surveillance" for '50s spy movies and Banksy murals.
More generally, what about the chilling effect on legal and legitimate conversation?
...or your association with "Occupy" or some other political protest movement that someone in power disagrees with, or that your wife bullied some politician's wife for two weeks in school, or that your interfering neighbour with a petty dislike of how you landscape your garden works as a government clerk and can access your data.
There are many reasons why some individual might want to know private things about some other individual. When individuals with some tiny (or vast) power want to wield it over anyone else, especially when they can do it with little oversight, it's very tempting.
That "the government" has access to my private information does not mean it's blind and faceless. It's made up of people with complex motivations.