Take this example:

  for (i=0;i<n;i++)
    A[i]=0;
  for (i=0;i<n;i++)
    B[i]=0;

It can be conveniently transformed into this:

  for (i=0;i<n;i++)
    A[i]=B[i]=0;

They are exactly equivalent except if the first loop never terminates.

Now, the compiler could try to understand if the first loop does or doesn't terminate, and apply or not the optimization accordingly, but Turing tought us that is indeed a hard task!

Or it could decide to never apply it, for fear of those rare and usually pathological cases where the first loop doesn't terminate.

Or it could decide to apply it by default and accept that in those cases the program does something different than what the source code says. The latter is better known as UB.

The third option won, and that's why infinite loops are UB in the standard.