It's also there to force the user to watch the full video to see if the build up to some final product was actually worth seeing. Both reasons are there to take seconds off your life to juice their engagement stats.

When I complained to my friend at Instagram about this, this is the exact answer she gave as to why they have it like this.

Instagram is a Duke of dark pattern ux, I'd say king but there's travel booking websites out there that hold the crown.

Given that Youtube copied it for their "shorts" clone of TikTok/Reels, it must be working for the platforms...

You definitely can jump in a video (at least on android).

The white bar at the bottom, which shows the progress of the reel/video, can be dragged. It is a very buggy experience, but it works.

I think it doesn't work with very short videos.

That's them trying to rate-limit the use by sex workers. Entire sites have sprung up to turn your single allowed Insta link into a number of other links.