Why is embedding images so inconsistent?

Sometimes it works with just a bare link, sometimes the link needs a [/img] or [] at the end, sometimes a gif gets a "play" button in the center but may or may not autoplay anyway, sometimes a video embeds like a gif but other times not, images from wiki sites with revision control are a tossup, some sites don't work at all (like Tenor and Giphy). And the live preview doesn't show the result (nor text color/highlighting), so the only way to know what your post will really look like is to post it.

What is up with this?