Probably to cut down on load times. Some people spam threads with gifs, videos, or high res images which can kill load times or even stall out the page from loading. And when I say spam I mean like one person uploading a ton of stuff instead of just giving links to a gallery or download page. A prime example would be the
You must be registered for see links
thread. I mean I'm grateful for the free content but when people spam it like that it's kind of obnoxious.