The same as any other forum I guess, same reason why there are gardening forums or music forums. People like discussing stuff on the internet... more often bitching about it but still.... and it's better to do that with people who share the same interests as you and therefore you overtime develop a liking for it.
In some respects sites like the Escapist operate as a community, it doesn't always get along but generally there's a friendly atmosphere here and it's fun to just have crazy conversations with people.
Also, I do still believe that with all the time and effort that it takes to make really great games, I'd still call them art.