The thing about UB is that many optimizations are possible precisely because the spec specified it as UB. And the spec did so in order to make these optimizations possible.

Codebases are not 6 lines long, they are hundreds of thousands. Without optimizations like those, many CPU cycles would be lost to unnecessary code being executed.

If you write C/C++, it is because you either hate yourself or the application's performance is important, and these optimizations are needed.

The reason rust is so impressive nowadays is that you can write high performing code without risking accidentally doing UB. And if you are going to write code that might result in UB, you have to explicitly state so with unsafe. But for C/C++, there's no saving. If you want your compiler to optimize code in those languages, you are going to have loaded guns pointing at your feet all the time.