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.

Squatters could move in the day after the property becomes empty. Really it depends on when it is noticed the house is unoccupied.
Sometimes houses can't be sold for months because of legal BS (happened with my moms house).

Counterpoint: some people would rent an Airbnb and stay after the two weeks they rented, effectively preventing the homeowner to return to their homes after a vacation. There's little legal recourse to speedily remove them, as two weeks of occupation requires a lengthy judicial process to evict them (IIRC in California).

I dislike rent seekers too, but it happens to people with only one home as well. They think they could put their home to use while they're not there (effectively reducing the problem of real estate under occupation), only to be exploited.

For there to be squatters, the landlords had to have this property open and unrented for a while.

Huh? A squatter is most commonly simply a former renter who stops paying without moving out. The property is not vacant at any point.