One of the suspected main reasons why we don't have proper original Xbox emulation yet on the PC is because of the fallacy that "it should be easy to emulate an x86 console on x86 PCs". This has lead to many failed attempts to attempt "shortcuts" of running Xbox games on PCs, like converting executables statically, or extremely high-level emulation approaches like only emulating graphics calls. None turned out not to be so easy in practice. The notion "it should be easy to emulate architecture X on architecture X" does not tend to hold up very well, at least not for game consoles that also have complicated graphics and sound.

Compare this to Dolphin, which has been wildly successful in emulating the Nintendo GameCube. Besides the big popularity of Nintendo games, the GameCube was comparable in complexity and sales figures to the OG Xbox. But since it had a relatively non-standard CPU architecture and a non-standard GPU setup, it guided emulation developers to not put too much effort in shortcuts and tackle the emulation problem head on.

It's worth noting some recent progress has been made in OG Xbox emulation by the Xqemu and Cxbx-reloaded projects. The former tries to use qemu for x86 emulation (or even virtualization?) while "low level" emulating the rest of the Xbox hardware, whereas the latter started life as an extreme high level emulator that is going more and more low level over time.