[notahacker]

It's pretty much the default position if an actual vacancy exists but then gets filled; unless your mailbox is being overwhelmed by new applications, taking down the job offers takes more effort than leaving it up.

Many companies have more or less phantom job listings on their websites and the free boards because whilst they haven't got any positions that need filling, they can afford to create role(s) for the right candidates. And as you've said, some probably can't afford that yet but at least they look like they're growing and get the resumes of people interested in what they might be able to offer in future.

Larger companies often have job ads up for positions they expect to fill internally, because policy requires they advertise it externally.