But that's at a cost of giving up knowledge of whether it overflowed. In this case it overflows and underflows, when it does, an equal number of times, so there happens to be no problem. In some cases, all you care about is the answer modulo some number which jives well with your numbers' representation. In that case, there is also no problem.

In many cases, if there is a different number of overflows than underflows, you'd get the wrong answer without knowing it, and your answer is likely (but not guaranteed) to be very wrong when you do.

There are other approaches we can take. Overflow could saturate. In that case, we get the wrong answer in one direction here but it's only off by a little instead of a lot. Overflow could also trap, in which case we'd notice that we had risk of giving the wrong answer.

In either of these cases, we could not apply this optimization unless we allow for some measure of undefined behavior - which is why undefined behavior is so useful. It is absolutely the case that undefined behavior in a specification has significant drawbacks as well - but the issue at hand here is not the result of them.