I'm guessing that it is partly because the Lemmy clients don't have good integrations for showing the gifts and videos inline without having to click through yet. Once that gets better I would expect to see more of that kind of content here.

For now, the size of videos probably means only links to video will be supported. See here for a better explanation: https://lemmy.dcrich.net/post/1069

I've been trying lately to post a picture but think linking the source in the thread.