Who do so many people buy a PS3, but end up playing nothing else but Call of Duty?

This is something that is annoying me for quite some time now.
And i just dont get it.

"Who do people buy a PS3, with TONS of great exclusives, and only end up playing CoD?".
Many friends on my PSN (including real-life friends) only play CoD.

When i look at their trophies, the only trophies they have are the following:
Modern Warfare 2
Call of Duty: Black Ops.

This is getting so annoying!
Why buy a PS3 if you play nothing else but CoD? You are better off buying an Xbox360 then, because thats the good version of Black Ops. Or just play it on PC!

One lame example was yesterday. I convinced someone to download the open beta of Killzone 3. He downloaded it, installed it, and played it with me. We both had our mics on!

So i asked him: ''So you like it?''

His answer: ''Yeah its cool. But i got a 5 kill streak, and where is my reward?''

what? i didnt even respond to that question. Unfortunatly, an hour later, i had to go to work. Came back after a couple of hours, and guess what? He was playing Black Ops again.

I just dont get it. What is so special about Call of Duty? Im not trolling about CoD, because i think its a pretty decent game. But not THAT great, so that everybody has to play it 24/7.

Hell, everywhere i go (School / Work) everybody is talking about Call of Duty.