Not to mention the problem is pretty eminently solvable: let users flag a car as having an unacceptable condition. If that happens dispatch a new car and send the dirty car directly back to home base for cleaning.
The main problem with Uber/taxi quality is that the responsibility for cleanliness is on individual drivers, with widely varying results as a natural outcome. In fact a lot of problems with Ubers and taxis is downstream from the ownership and responsibility model.
The advantage of something like Waymo is that the responsibility is now on Waymo itself.
Worth noting that this is a problem in other lines of business: it's harder to ensure quality in franchises vs. stores operated directly by the brand. The more independent parties you have in the mix the more incentives become misaligned and the fewer levers you have available to ensure compliance to some standard.
This also isn't impossible to solve with human drivers, because ultimately this isn't a technological problem but an organizational one. Livery car services where drivers are employees (as opposed to independent owner-operators contractors) can centralize cleaning and training, and have more means to ensure compliance to a standard.
The "downside" of such a model is that there are many more laws to ensure you can't shovel your own expenses onto the employee.