These weren't easily accessible, unlike DVDs or divx.
I do not see what is your point here.
There are a few hundreds of thousands of pictures on the site. Replacing movie captures by HD versions take a lot of time.
And since many of us add pictures in non-HD, that's an endless task.
Also, in many cases the current non-HD pictures are already good enough, it is not like if all pics on the site were from VHS tapes.