Because that's hardly ever what happens, except when it actually does, and compilers do an increasingly good job of issuing diagnostics in that case. If you actually mandated it, no compiler today would come close to being standards compliant. This comes close to making the language unimplementable.

The most common issue with UB and optimizations is not that "compiler detects UB and does something with it," it's that compiler analyzes and optimizes code with the assumption that UB doesn't actually happen. It doesn't know whether it does (and in general, it is impossible to tell whether it would happen -- it's something that might or might not happen at run time, and proving it one way or another amounts to solving the halting problem), it just assumes it doesn't.

And if one mandated compilers to report every time they make an optimization that is valid under the assumption that the program is well behaved, then you would never finish reading compiler output. Or you would turn off optimizations.

    find_in_array_loop:
    for(i = 0; i < n; i++)
        for(j = 0; j < n; j++)
            if(array[i][j] == x)
                break find_in_array_loop;