YouTube shorts does have its own progress bar, but it's proprietary and kept out of view and so it doesn't allow the user to tell the length of the video at a glance.

I've noticed the seek bar is available in MOST but not ALL shorts. I always thought it might be an option by the video owner.

I'd say it's more likely youtube A/B testing to see if they can increase engagement with that bar disabled.

Tiktok used to do it based on length, is it something like that? I've noticed having it on desktop more often than mobile on YouTube shorts, maybe never on mobile browsers.