Well, they may really think it does have the best games - but that doesn't stop them from wanting more 
You can call it greed or whatever, but people just want to play all the games they can. I'll bet many PS3 fans are hoping SO4 or ToV is announced multiplat, that's just how it is. It's only prevalent now (as opposed to last gen) simply because there's a chance of it happening, however slim (though with so many going multi, it doesn't seem that far fetched).
We've never had internet before with a console race as close as this one (erm, without the Wii), because the PS brand has always hogged all the third party support. For all we know, it may have been like this in the SNES/Genesis era if we had internet, no?
edit: It should be known that the PS3 has always had a better "future lineup." However, with the rising 360 userbase, they've been able to snatch quite a few titles... yeah, why stop at third parties? Why not God of War 3? (lol)
