The only way that happens is if the rent is too high.

That’s not the only way. It’s not even very likely. If they are looking for too much rent and can’t get it they will lower their ask rather than sit there month after month getting nothing. Too high rent is the most easily fixable situation conceivable.

Other explanations include things like: it’s owned by someone who is elderly and due to their health or other problem they simply aren’t managing it actively or are even incapacitated and can’t make major decisions. Perhaps the owner died and the property is in the probate courts, which can take years.

Also, the presence of squatters doesn’t necessarily indicate it has been vacant for a long time.