There is no reason to assume that people driving cars they don't own are as careful as driving their own cars. This makes a huge difference.
A few years ago, they even stopped asking customers at the start of the rental whether there were any “new” damages with the car, since it's impossible to verify that with dozens of damages, and people would just press “No”. So now these cars all have small damages that nobody cares about, which accumulate until they get repaired.
a) My main worry is hurting someone else; I will be responsible for this either way.
b) If I do damage the rental, chances are I'll be held accountable at the end of the rental, and chances are the repair fees will be extortionate compared to fixing my own car at a shop of my choosing.