1. “Rubber” bullets are a massive metal ball wrapped in a thin sheet of rubber. They carry about half the kinetic energy of a bullet, they can still crack your skull or destroy a part of your body that they hit. There is a reason people started pushing for the terminology “less lethal” instead of “nonlethal.”
2. That said you actually raise kind of a good question I think. I suspect that a lot of the reason is nothing more than that the guns that shoot “rubber” bullets effectively are big and cumbersome. You can’t run fast while holding one, carry one around on your belt and then pull it out in a fraction of a second, et cetera. They actually do try to do what you’re talking about with Tasers, there’s a whole process, except that Tasers are unreliable so they have to have a second cop with a gun drawn most of the time.
3. Replacing guns with “rubber” bullets… a lot of the time when they are shooting they are thinking in terms of a gunfight with an armed suspect, so they don’t want to be in a situation where the “rubber” bullets aren’t penetrating a car but the bullets coming back at them are penetrating their car, something like that. If it is deadly force involved they don’t want to be at a disadvantage.
4. Replacing Tasers with rubber bullets… IDK, I think “rubber” bullets are probably more lethal than Tasers and you’re definitely going to fuck somebody up any time you hit them with one. The vast majority of the time, the Taser just sucks and then you take the probes out and you’re done, you don’t have any cracked ribs or destroyed eyeballs or anything. Most of the scenarios where they would be using a “rubber” bullet, US cops at least will use a 40mm “beanbag round” which won’t cause nearly the same type of injuries.

I won’t say your suggestion is automatically a bad idea but I think those are some of the reasons you so rarely see them except in “crowd control” type of scenarios where some of the existing nonlethal options aren’t viable, and also where they have some additional desire to cause injuries in the people they’re “control"ing. Basically you can choose a Taser which is unreliable, pepper spray which is short range and will fuck you up too sometimes, or a 40mm or rubber bullet which needs a big cumbersome launcher (and the “rubber” bullet may cause significant injuries anyway).