Why sedans don't have rear wipers?

Most sedans do not have rear windshield wipers. Most, if not all SUVs, on the other hand, have rear windshield wipers.

Why is that?

ask the manufacturers. My guess is clearance issues boot/wiper arm

If you Google what you have just written , you may come up with an answer like so .
https://www.quora.com/Why-do-hatchbacks-have-a-rear-window-wiper-but-sedans-dont

They sometimes do.

Common to see on Japanese market models like the Legacy, Skyline and Atenza

on an suv or station wagon the airflow over the rear of the roof tends to 'drag back' dust, dirt and moisture directly on to the rear window. Airflow is smoother over a sedan window and tends to blow/wash stuff off. The angle of the window on a sedan means it gets rain directly on it.That's my theory anyway. Anyone remember the dust deflectors on s/w's in the 70-80's? My basic nissan s/w has no rear wiper and it's a pain.

Never had the need for one & we don’t even use the one on my wife’s hatchback.
We wash our cars weekly.

why would you want one?

This is most of the reason, as is the fact that room has to be found under the rear shelf area to accommodate the motor, which compromises the overall depth of the load area.

I have an A33 Nissan Maxima which has a rear window wiper, and I use it alot, especially on those misty drizzly days or those evenings when dew or frost can cause the back window to become difficult to see through. I agree that dirt on the back window isnt usually an issue because of the way that air will flow over a sedan keeping it clean, but for the issues I mentioned, it is good to have that extra rearward visibility from the moment you hop in the car rather than waiting until you are driving down the road and relying on the airflow over the car to clear the moisture off the rear window. As far as compromising boot space, the wiper motor protrudes less into the boot than what the side and centre seatbelt reels do. On a previous Maxima with the rear wiper, it meant the 6 disc CD changer was re-located to under the passenger seat rather than under the parcel shelf like the non wipered models were. Again, the wiper motor took up less space than the CD changer. I understand that there could be a space/access issue with a smaller sedan, but there are many sedans that have CD changers in that position without being an issue.
https://trademe.tmcdn.co.nz/photoserver/full/861180526.jpg