My thought (as a non techie) is that it's just easier to ask for everything.

Need to use an open source library in building your codebase? No need to analyse everything to see which permissions it does or doesn't impinge upon. Want to add a new feature down the track? No need to consider whether it changes the user permissions.

And I'd be surprised how many people are turned off by the number of permissions - I'll almost always back off if there are any, and if I really want to use the product I have to trust them to do no evil. My guess is an awful lot of people are always living in that trust space, especially if they don't understand the tech.