inb4 "because we're running mainly on MS software"
;)
-- edit --
why the down-votes? I read about other websites, like POF, who run on MS software and their server-architecture is monolithic, too. The time I read about it, the page run on 2 big Xeon machines.
If you are actually interested, here's a report one of our sys admins did a couple years ago estimating how much it would cost to have us running on "the cloud" - http://meta.stackoverflow.com/questions/73969/what-would-sta...