Register variables, why not?

Hello. I have been studying C and C++ recently, in order to build deep knowledge of them and thus be able to write good C++ code for Unreal, and I came across register variables.

Apparently, in C and C++ you can declare a variable as "register" if it's used often, and it tells the compiler that said variable can be saved directly in a CPU register so that it can be accessed faster, since RAM is "slower" than CPU registers, so I immediately thought if I could use them in gamedev for frequently used variables, e.g. health, to squeeze some extra optimization.

Of course I'm aware I am no genius of programming and that the answer would 99.9% be "no", because 1) I don't know if they are even supported by UE and 2) if it was the case it would already have been done even before 3D games. I am curious tho as to why exactly it wouldn't be a feasible idea, what are the reasons exactly that would make register variables a bad choice for game variables?

The only things I can come up with is that they would be very volatile and could make saving trickier, besides posing some possible problems for replication, but I am curious if there is something more I don't know