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.
That doesn't cover all the normal cleaning needed just from people being in the cars, on top of environmental stuff (sand, dirt, mud, leaf debris, pollen...)
Like others said: the second they have run drivers out, the cars will stop getting cleaned obsessively because you won't have a choice.
Same thing that happened when Lyft and Uber when they ran the taxi industry out of business.
Same thing that happened when Zipcar was established. Cars went from being spotless and well maintained to damaged, dirty, and half the dash being lit up.
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.