The only reason UB exists was that when ANSI C came to be, the compiler writers and hardware vendors that provided C like compilers didn't want to give up on their semantics.
So in order to please everyone, all the semantics that could not be defined on the standard became UB.
For instance, what should `sqrt(-1)` be? Or `head([])`?
You can define all behavior and say "bad input will throw an exception" or "bad input will abort the entire program" but who really cares what it does since it's a malformed program in the first place? Just say it's undefined and it's up to the programmer to make sure it never happens. Otherwise you're littering your code with countless checks for things that should never happen in a properly coded program.
Programmer error is distinct from operational error. There is no sane thing to do in a program error. Constantly checking for programmer error at runtime is wasteful.
Undefined behavior though?