How come when I see a review for a PS3 game someone says...

"I bought a PS3 specifically for this game"? (in comment sections)  or "If you bought a PS3 because of this game..." (in reviews) and I've seen this for recently released games like "Heavy Rain" and "God of War 3". They're making it seem as though the PS3 has no good games worth buying AT ALL.
 
Notable PS3 Exclusives: Metal Gear Solid 4, Infamous, Resistance 1 & 2, God of War 3, Heavy Rain, Uncharted 1 & 2, Killzone 2, LittleBigPlanet, Ratchet & Clank (both games released),
Notable 360 Exclusives: Halo 3, Halo 3:ODST, Crackdown, Gears of War 2, Fable 2, Shadow Complex, Alan Wake, Splinter Cell (I left out games that are also on the PC)
 
I have both a 360 and a PS3 but I see more attention paid to the 360. And for the record game that made me buy a 360 was "Dead Rising" and the game that made me buy a PS3 was "Metal Gear Solid 4"
 
 I've never read anyone saying recently that they bought an Xbox 360 just for "Alan Wake",  "Splinter Cell" or ODST. Has anyone else noticed this? 

It's just a function of the install base and the fact the PS3 started later then the Xbox 360. 

I own a PS3 and Xbox. I bought my PS3 because I was sort of fed up the way MS handles their Live services, and because Metal Gear looked cool. I regret buying my PS3 big time. Not saying they dont have any good exclusives, they just don't have any that interest me.

I own both. I have bought the 360 last week. Mostly for Shadow Complex, Splosion man, Braid, Perfect Dark and some other XBLA games.  
So not really the "big titles". I will pick up Halo, Fable and Banjo. But only when I've played most other games. For me the PS3 exclusives are far more appealing. Games like Heavy Rain, Ratchet & Clank and God of War make me happy with my PS3. (Mine has BC, which was a big pull as well, but since they removed that and you can play those games on PS2 I'll leave that out of the argument.) 
 
Since I just own the 360 I can't really say yet which controller I like more. The UI is a lot simpler for the PS3. Of course the 360 handles online far better. Honestly I can't say yet for which console I'll buy the next multi-platform title. 
 
On the other hand, when it first became clear that Bayonetta plays better on 360. The internet was ablaze with trolls and fanboys laughing at PS3 owners. With FFXIII I haven't seen that as much. So perhaps PS3 owners are less vocal? 
 
As for saying "I bought a PS3 specifically for this game" I don't really see how that says the PS3 doesn't have good games.
 I bet most 360 owners first bought it specifically to play Halo 3.

If it makes you feel better I bought mine mainly as a Blu-ray player and now I prefer it over my 360.

" when it first became clear that Bayonetta plays better on 360. The internet was ablaze with trolls and fanboys laughing at PS3 owners. With FFXIII I haven't seen that as much. So perhaps PS3 owners are less vocal? "

Well, what can you do? 
 
Even though it's not true, the Xbox 360 has cemented itself on the chair that says "everybody should own this console". The PS3 still has to catch up in sales, so it's much more likely people own 360's than PS3's.

Chances are, if you're regularly using a site like this, then you probably bought a 360 a long time ago and did only buy a PS3 for the exclusives and/or Blu Ray.