…because it actually affects peoples lives, and not in a positive way: studies have shown that if people believe they are being observed, then they tend to alter their behaviour to match what they think the observer wants to see. I want people to be able to do their thing without fear of consequences from bigots or The Man or even “ordinary people”. None of us are ordinary and the world will be a poorer place if we were made to be.
I've seen this argument come up a couple of times recently but it seems to be seriously underrepresented in my opinion. This is the key response to the "you only care about privacy if you've got something to hide" argument. Essentially, we all have something we hide from various people and even if we don't mind them finding out, just the fact that we know someone is watching makes us behave differently.