I think the difference is that as an individual driver you have an incentive to keep your car clean, so that Lyft continues to dispatch riders to you. For Waymo the selective pressure is less direct and also spread across their entire fleet. They can accept a level of dirtiness, given some probability that the rider would reject the car x cost of rider requesting a (partial) refund x etc. etc.

More cynically, there are simply too many people that won't take care of "public" property. If every 3rd rider (exaggerated for rhetorical purposes) trashes the car, it's gonna be dirty no matter what.