Why do you find it surprising that a lot of people have been playing since CoDs before CoD4? CoD2 was one of the first shooters for the Xbox 360; I'd imagine a lot of people who got the Xbox 360 when it first came out ended up buying CoD2 as well.
The community grew exponentially after Cod4 was released. It's hard to imagine now but before Modern Warfare CoD was perceived as just another shooting game, there was an average sized community and Halo was infinitely more popular.
Oh trust me, I know. I'm just saying, being that we're on a video game forum, I'm sure a lot of people here got the Xbox 360 close to launch...and at that time, the major shooter out for it was CoD2, so I don't think it's too farfetched to think that a lot of people here started with CoD2 as well.
CoD2 for single player (****ing awesome 360 launch title) and CoD4 for multi. CoD3 sucked, I shoulda learned my lesson about Treyarch. I skipped WaW and shoulda skipped this. BO1 had good multi tho (still awful campaign).
A lot of people here also list CoD4 as their starting point, topics like this get made all the time. Not to put them down though, 360's launch may not seem that long ago but in gaming years it may as well be an entire player generation removed. The kind of crowd that hung out in CoD2 lobbies seems to have been lost for a while now.