Implementing this over USB is a lot more complicated than bit-banging SMBus with MMIO. That means you have to learn how modern PC hardware works and you have to think through a protocol design, think through access control semantics, God, maybe even design an API.
The people writing these drivers are the people making these devices and they are not operating systems people. These devices were developed and prototyped as bit-banged gadgets built around standard simple microcontrollers running on standard electronically-simple I/O buses. This is how stuff was done on PCs too back in the MS-DOS/Windows days.
This whole segment of the hardware industry got into a snag when Windows XP came out and the home PC world started moving to NT because that meant they had to start thinking about scary things like memory protection and stop treating the NT kernel like an Arduino. So, from the very beginning, they'd find these niche hacky drivers that let them bit-bang portsâ"yes, usually they just found something some small firm wrote to solve a specific problem and they shared them with each otherâ"and they'd continue writing their control apps to bit-bang the chip directly. And because this area of computing is adjacent in many ways to the world of niche industrial computing, which is where these hacky drivers usually came from, Microsoft signed the drivers supported it just enough that the show went on.
The kernel team, being the most competent engineering team in all of Microsoft, likely knew that getting these people to learn kernel programming was a pipe dream and they've done a good enough job of blast-proofing the NT kernel around these hacky drivers, no one ever really had to point out that the 800 pound elephant in the middle of the room is still writing hacky VxDs for Windows 11.