It might be that the vacancies are just there to create the illusion of growth. Because if you're hiring then you're clearly growing. I recently interviewed with a company that straight-up told me that they don't have any positions right now but might in the future. That same position I was interviewed for got re-advertised a couple of months after my interview.

Come to think of it, I know of several examples(startups) where the company was actually failing and about to run out of money and they still advertised but couldn't possibly hire. In my opinion, that's purposefully wasting people's time.