because you can just put your mouse right on the head of an opponent (takes no real skill just practice in mspaint)
i feel like no one even talks to ech other on a pc because A you are just playing to hit on girl gamers or B you are there just to look cool and really not even into gaming, ive been playing halo for years and still have all of the same people on my friends list, and on the xbox they always offer exclusive downloadable content like new maps, avvatar clothes, etc. that are a really fun way to keep the same game new and exciting for the community as a hole