So why is it not the case here?
It's not the case here because the PS3 already has a bunch of good games. Good PS3 games haven't stopped being released due to the alleged perception that gamers don't think they exist. In fact, the number of good - hell, great - PS3 games is constantly increasing, regardless of any sort of negative perception on the part of the gaming public. However, I'm guessing that a lot of people on the NWR forums don't own a PS3, so they have no reason to care or be educated about its better titles.
On the other hand, you have the Wii, which is currently getting plenty of garbage shovelware, party games, and "casual" titles, because that's what publishers believe Wii gamers want. Now THAT is perception shaping reality. For instance, do you think that Activision created its "Wee 1st" game label thanks to some consumer write-in campaign? No, it's because they believe that there isn't much money to be made in hardcore games on Wii. People on this board would surely say otherwise, yet due to publisher actions like this we're sure to see fewer hardcore games on Wii. Activision's perception of the market has thus shaped the reality of the market.
I'm sure the PS3 has several decent games, like the 3DO and the Jaguar, even.
Congratulations, you're condescending.
most sequels on the system selling less than their previous gen counterparts (GTAIV for instance. Much less)
GTA IV sold "much less" because the PS3 userbase is smaller than PS2. However, in terms of tie ratio, I've read that a slightly higher percentage of PS3 owners bought GTA IV than 360 owners. *shrug* If you can come up with another example of a sequel to a PS2 game that was some sort of disastrous failure on PS3, I'd love to hear it.
Sony has actually taken a lot of risks with their PS3 library by relying on several new IPs (Motorstorm, Resistance, Uncharted, Folklore, Eye of Judgement) to carry their flag, instead of relying on sequels like Nintendo (Mario Galaxy, Brawl, Mario Kart Wii, Metroid Prime 3) or Microsoft (Halo 3). If Nintendo relied on Zack & Wiki, No More Heroes, and Okami, people would probably assume that the Wii had "no games of value" either. And of course, they would be wrong.