Why have you not purchased Uncharted? Ratchet & Clank? Heavenly Sword?
The quick answer? Because both of my PS3's were bought to be movie players.
The longer answer? I have played all three games, just the demo for HS and R&C, and I got Uncharted free with a PS3. I don't think anyone will dispute the high technical merit of these three games, but going by gameplay alone it breaks down like this for me:
HS: This may sound ridiculous, but I can't play this kind of a game without the ability to jump. Plus, I always felt disconnected from my character. In God Of War I felt like the character was an extension of me. In HS, I felt more like a director, suggesting what should happen and waiting to see how she would respond. I hope that they just copy the God Of War formula for part two, then this game could be exceptional.
R&C: Fun for a bit, but just not for me. At the risk of offending 95% of this forum, it just seems like a kids game to me.
Uncharted: I'm 49% of the way thru this one. It's a good game and fun to play. The problem as always for me is that I keep comparing it to Tomb Raider Legend which to me is better (gameplay wise, definitely not visually!!). Tomb Raider had a better story, tons of varied locals, better puzzles, and was less of a run and gun type game than Uncharted.
Two more points to mention. If you have a 360 as well as a PS3, then the above games get beat out because the 360 just has so many good games available for it. So of the 11 million or so PS3 owners out there, how many don't have a 360? They will be the best candidates to buy the above games, the rest are probably buried in the 360's amazing catalog of games this year.
The last point, which will also seem ridiculous to many, is that I honestly find it hard to play games that don't offer achievements. Yup, sounds crazy, but while playing Uncharted it would sometimes pop up rewards or unlocks for doing certain things, which to me seemed so pointless compared to the global achievements system on 360 games. I admit I get a perverse pleasure whenever that achievements dialog pops up while I'm playing a 360 game. I know its part of a global system and ends up on my gamercard, which is awesome. It's hard to play games without that now. Silly? Perhaps, but hey thats how I feel about it.