[crote]

Of course, but there's no reason why a 50/60Hz signal couldn't have been included in the ATX power connector back when it was established a few decades ago.

In an alternate universe it would've been put in there, together with all the weird -12V / -5V rails nobody uses these days. Getting it these days would indeed be pretty much impossible.

[labcomputer]

Sure, but it costs extra and nobody (to a first approximation) cares. So why bother?