This is a great question.
I think the crux is not so much your equity but how often you are going to realise your equity - in other words, even when you flop decently well, how often are you going to make it to showdown?
Suppose the flop is T62 rainbow, you bet and get a couple of callers. This would be a great flop for you if heads up, but in a 4-way pot your opponents, even if they have missed the flop, will almost certainly have at least overcards and gutshot draws between them. So unless you hit another ten or an ace, on what turns and rivers can you profitably call down if you face aggression?
Some other possible factors:
If you flop the nuts with a KQJ board, how often are you going to get paid?
If you flop a ten and a dominated opponent has, say, QT or T9, how often are you going to get paid when an overcard comes on the turn or river? When it does, an opponent could easily have improved to two pair. When it doesn't, there are now lots of straights out there.
Multiple opponents could easily have pocket pairs which are set-mining. With two such opponents, the odds that one will hit a set are roughly 1 in 4, so you face significant reverse implied odds the times you continue and end up with a strong pair or two pair vs a set.
Multiple opponents could easily have suited cards, so any two-tone (or monotone) board is bad for you.
As you have noted, even an ace is not great for you, since your oppoents could certainly have AJ or AQo (and the HJ at least could also have AK or AQs) - more reverse implied odds.
A decent chunk of your equity with a hand like ATo comes with the fact that some of the time you will improve (hit an ace or ten) on the turn or river and overtake an opponent's weaker pair. But with multiple opponents, someone will probably flop something, so it will be much harder to continue and realise that equity when you miss the flop than if you were heads up.
- Against multiple opponents it is much harder to get to showdown with anything other than a monster hand.
- When you do hit with ATo you are going to struggle to get paid.
- When money does go in, even your monster hand could be beaten, causing you to lose a massive pot some portion of the time to, say, a set or flush (probably more often than you will win a massive pot).
This is more of a comment than a genuine answer, and I'm sure there are other relevant factors, but it was too long for a comment so I will post it anyway in case it's helpful.