You'd struggle to get games like this with the financial backing it's had from its publishers without first party support. Certainly not without a lot of compromise, not least down to the modern day cost of making a game, let alone making two versions for each hardware setup. Plus there's the fact that multi-platform developers tend to go for mass appeal, rather than knowing that they can focus on a more specific type of gamer.
And of course it means that the hardware of each console is not being used as effectively as possible. Look at FIFA of late - 09 ditched the pre-match amble about derby games to make space on the disc, EA insist they can't patch 10 because of restrictions with patching games on the 360, and if I remember rightly the lack of a hard drive on X360 Arcades has been part of EA's excuse for lack of patches and hard drive replay saves as well. None of these would have been a problem for PS3 users but because of the modern day politics of not upsetting the users of one console by giving them significant feature advantages over another, we are missing out.
I can't think of (m)any high cost third party games out this generation with the same level of ambition and creativity as games like LittleBigPlanet and Heavy Rain, and I'm sure X360 owners would say the same of games like Fable.
Obviously there are generic exclusives, and generic first party titles - generic titles are often based on high-selling predecessors - but when third party studios have been playing it as safe and multi-platform as they have this generation, and there's the extra incentive for the publisher of using software to try to shift hardware, there's more of a need to make something unique to get people picking one system over another.
For the record, I don't disagree that the games selections are too similar to warrant owning both consoles. Hopefully Natal and PS 'Arc' will take hold enough to make the two systems diverge a bit more. I've got a monstrous PC and a 360 pad, so I'm happy provided MS don't pull the 'well the PC isn't cinematic' PR bullshit they did with Alan Wake.