Explanation
The mechanic is that swiping is an interaction and watching is not. A viewer tapping through six slides generates far more engagement signal per second than the same viewer watching a video go past, which is why a format with no production value at all keeps beating things that took an afternoon to make.
It is also the cheapest thing an account can post, which makes it the correct format for testing whether an idea has legs before committing a full video to it. Slideshows travel to Instagram reasonably well and to YouTube Shorts badly, because Shorts is a video surface and a slideshow there reads as a video that forgot to move.
The common mistake
Putting the payoff on slide one. The whole format depends on the last slide being the reason to swipe through the middle, and a slideshow that front-loads its conclusion has removed its own mechanism.