Because information is power, and I don't really want the government to have that much power over its citizens. Sure, I have nothing to fear... from a
just government. But the government is not always just, and the more power they have the more potential for abuse of power.
I don't have anything wrong with warranted searches and seizures though, because that's reasonable. What's not reasonable is the government being able to know someone's location, who they've been talking to, etc. without a warrant.
Here's a demonstration of what they can do with your data. And this is without a warrant.