Halo: Spartan Strike and Halo: Spartan Assault are on Windows (Steam even).
I don't think they ported any of the FPS titles since Halo 2 because Microsoft is not going to bother putting in the months of development time to successfully port it. Additionally, Microsoft makes money on Xbox 3 ways: the game purchase, Xbox Live subscription, and the console purchase. Windows is a mighty gaming platform but it just can't compete with that kind of revenue from Xbox. This is also why Sony Entertainment America releases next to nothing on Windows too (too busy milking the PlayStation cash cow). Nintendo too for that matter.
Look at SEGA and Atari, on the other hand. Both have titles on virtually every platform.
Frankly, I think we need international regulation on this stuff. If a company is going to make a console and they are going to publish titles on it (which they all do), they should be forbidden from developing their own titles as well in order to foster an open market and prevent publishers from buying out developers (would force the opposite: publishers to spin off developers). This mentality should extend to all digital markets (app stores, video distribution services, game distribution services, and so on). The market is always skewed in favor of the market owner.