Because it actually sucks. Pages and tabs are much more useful.

I expect a "sow, then reap" (LOL) pattern is super-common on these sites: browse one or a few searches or categories, new-tab anything interesting and move on, then close those and look through what you opened when you think you've got enough queued up. Not just scrolling down a single "feed".

Refreshing the page means new ads and there's another opportunity to put a floating banner ad and popup-overlay ad in front of you.

imagine reloading the page during a cliffhanger and losing the video you were watching

Lol porn is addictive enough!

I also wonder why TikTok style infinite scroll auto play isn't an option

redgifs dot com

They're optimized for short form, continuously scrolling, mini videos. They don't do the recommendation algorithm though, at least not for free users.