Third party titles aren't exclusive for a reason. They care about maximizing market availability across platforms. You can say that Gears and KZ look better than COD all you want, but what Activision will ultimately point out the total units sales across both the 360 and PS3.
COD is probably a bad example because it sells heaps regardless of technical merits, other games do not. And of course the combined market of two, or more, platforms is obviously larger than one of those platforms in isolation but if one platform version is lacking, it's more likely going to be by that platforms owners for platform exclusives which are better. Unless they own both consoles of course, where the option to buy the optimal version exists.
An platform exclusive gets the benefit of being allocated the total resources of the project. Multiplatforms don't get that benefit and pubs ultimately want the resources to be devoted in a way that leads to the overall benefit of the game not one platform specific port in an effort to look as good as some first party.
I'd argue it's far less about resources dedicated to development and much more about designing a game and engine that takes advantage of all of the strengths and none of the weaknesses of a particular platform. The moment you go cross-platform it's a case of limiting design and technology by LCD.
Plus its not artificially maintaining technical parity. That denotes gimping one over the other.
You can achieve technical parity without gimping by allocating resources to optimise one version over another.
Would you consider a 360 title gimped simply because a developer spent extra resources to push the SPEs performance to the point that it allowed the PS3 title to reach a quality similar to the 360 port? I don't.
No, because PS3 games often need assistance from the SPEs to balance the inadequacies of RSX compared to Xenos. This is optimizing the PS3 version, not gimping the 360 version.
Most have a goal in mind in terms of visual quality of the finished quality. If one hardware gets there faster or easier there nothing artificial about pouring additional resources into the other hardware to get it to obtain the same goal.
I posted exactly this above a few days ago.
These games aren't someone's pet project. What matter the most is how these games sell not how they look. Obviously visual matter but its not like you can tightly correlate the top five selling games on any platform with their visual quality versus other titles.
Actually, you probably could. What was the last chart topping game you saw with terrible visuals (art, technical execution etc)? Some might argue Minecraft but the graphical style was a deliberate decision.
I'd bet when it comes to multiplatform titles of two similarly performing hardware, most of the motivation to push the hardware comes from competing against the visuals of the other team not some outside first party exclusive.
I'm sure there is a degree of that and healthy competition of this nature is good, but the publisher only needs a sale, the platform is less important. That makes the dynamics of each platform's userbase, buying habits and market dynamic more important than the other platforms.