As another mentioned, it's not necessarily the only camera that might need wiping, but the front camera is the primary camera for AP/FSD. There's been tests done that AP/FSD can function even with all other cameras covered, but the primary camera must be active for it to work at all. This makes sense given how the system evolved from AP1 (which only had one camera in the front).
The other detail is that side cameras can be designed taking advantage of the aerodynamics to keep the camera clear. We can't see if that is the case for the pillar cameras, but it appears to be the case for the fender cameras. Theoretically, this should have been possible with the rear camera too (if Tesla had a shroud over it perhaps with hydrophobic coating), but Tesla failed to do so.
You can see this to be the case also for the Gen 5 Waymo. The top roof cameras have wipers, but the other lower cameras that face the side or rear do not. Instead they are shaped in a way that makes it unlikely for water to make its way in, and according to other articles, Waymo also uses hydrophobic coatings on some of them.
Sensing Breakdown: Waymo Jaguar I-Pace RoboTaxi. LiDAR, radar, cameras, ultrasonic, oh my!
BTW, I do not see a separate window for a rain sensor for the roof cameras, so I suspect Waymo is doing a similar vision NN to detect rain.