It kinda was though. Halo 1 and the Xbox is the reason online FPS on consoles are a thing. Not necessarily a good thing, mind you, but you can't deny that it did completely change the market.
It's not like loonix and wangblows. Not by far. With OSes you still compile for the same x86/AMD64 architecture and all you have to do is translate OS calls. The original xbox uses a fairly standard x86-compatible pentium 3, but all the other consoles have always used their own proprietary chips. Everything by nintendo uses MIPS-derivatives, sony uses their weird shit, sega uses some other RISC stuff from nipland, etc. You have to translate that shit to run it on modern PCs. Luckly, that's not actually too hard and as a result we have emulators for these systems.
The problem is that since 7th gen, all the consoles uses custom-made graphics cores in some form or another that are programmed through some API. In the case of microshaft it's DX-like, but for the rest it's all custom. Reverse-engineering those is miles more difficult, since it all goes through multiple layers of abstractions that eventually ends up inside a blackbox chip that does things that nobody other than the hardware designer knows about.
In the ole days when the consoles were all essentially doing software rendering with a few extras for acceleration (sprite engines, simple vertex pipelines etc.) it's feasable to reverse-engineer and port since you can usually figure out how it works by looking at chips by the same manufacturer and how they work. Example: even if the compiler for N64 is unavailable, if you can find docs for the chips the one in the N64 is based on you can work it out roughly from similarities and then fill the rest in by looking at official programming manuals and see what code should do what, then figure out what kind of hardware features it probably has and how they should behave, then you try to implement them, either in software directly or on top of another API, ex. opengl.
In the case of 360/xbone, since they're DX-like and they use PowerPC/presumably AMD64-like respectively and AMD graphics in both cases, it's not impossible that they might be reverse-engineerable in the future. We do have an open-source graphics driver for AMD, so unless they've made everything different it's possible there's enough similarities that a custom graphics driver for a custom version of linux could be made. Then that might be able to emulate all the services required to run stock games using a non-stock OS+software stack. If you can get that done, you might also be able to translate the machine code and run it on a PC whilst also implementing all the hardware features on top of other libraries and APIs (opengl for graphics ofc, but there's also a bunch of other stuff like input system, networking, dashboard services, sound system etc.).
As for PS3/4, Wii etc. I don't know. It's possible their libraries and APIs aren't too weird and I do know that you can run linux on the Wii, but xbox has the advantage that they're the most similar to what we have in the PC world.