a) Not just the Windows kernel - Office uses SAL, too, in numerous places. Mostly in legacy places predating more modern C++.
b) It's not true to say that kernel devs don't really use it. They use it a lot, though there are attempts at obsoleting some of it[0].
Oh nice, I didn't know that about office, it makes sense though. I don't work for Microsoft and mainly work in the Antivirus space, so I only occasionally hear stories from friends about their internal stuff. When I said driver devs don't always use it, I mainly meant third party vendors writing kernel code. I know alot of the overseas places don't, and I've rarely had jobs which use it.
Oh, I'd buy that in a heartbeat. SAL is, shall we say, most useful in the presence of non-public tooling, and it's also in general a fairly awkward language extension. Doesn't surprise me at all to hear that third-party devs haven't taken it up.
[0]: https://github.com/microsoft/wil