Who says they are? The spiders look at a number of things. What's in the title tags, the h1 and h2 tags, indexed links within, etc. It's just one of many things that add up all together.
Good question. I'm betting it's cause it was started a while ago before we knew the importance of SEO URL's.
Because it doesnt need it. The age of content, age of site, and backlinks far outweigh the benefits of mod_rewriting an established site with a large amount of posts and threads.
That and Shawn doesn't need anymore stress on the server than there already is. Using mod_rewrite to rewrite and direct the URLs would likely kill the server. As in "it's dead, Jim". D.O.A. R.I.P. Deceased. Kicked the bucket, bought the farm, went to the great server farm in the sky...