The compilers not doing array bounds checks is exactly exploiting UB to give faster code. If out-of-bounds accesses were not UB (ie they had a defined behaviour

The compilers not doing array bounds checks is exactly exploiting UB to give faster code. If out-of-bounds accesses were not UB (ie they had a defined behaviour) the compiler would be required to insert a bounds check in order to catch the case and ensure that the result was whatever the defined behaviour said it had to be. As you say, these days smart compilers can optimise to reduce the overhead of the bounds check; but the original 70s C compilers didn't try to be that smart.