Am I wrong in assuming that it's either difficult or a pain in the ass to create a functional internal search engine? My further assumption that Theymos isn't a slacker and would have one running that didn't have members going to google instead is a valid one I think.
My 2 sats:
It's very difficult to beat Google on search capacity.
But there's more to it: theymos is just one guy, and from what I've read, one of the reasons not to make many changes is security. Any change on the forum can potentially be abused, and if a third party (like Ninjastic.Space) can offer a similar service, at least that doesn't create an angle to abuse the forum.