> May have something to do with the fact that a big proportion of servers in production with many CPUs/cores are Linux servers so they started investing in this quite early?

Not servers, but single system image supercomputers. Quoting Wikipedia (https://en.wikipedia.org/wiki/Altix):

"At product introduction, the system supported up to 64 processors running Linux as a single system image and shipped with a Linux distribution called SGI Advanced Linux Environment, which was compatible with Red Hat Advanced Server. By August 2003, many SGI Altix customers were running Linux on 128- and even 256-processor SGI Altix systems. SGI officially announced 256-processor support within a single system image of Linux on March 10, 2004 using a 2.4-based Linux kernel. [...]"

And as I recall, back then Linux suffered an issue similar to what Windows is facing now: a single 64-bit integer was no longer enough for a processor bitmask when you have more than 64 processors. IIRC, a lot of code had to be refactored to allow for a bigger bitmask, and an abstraction layer was put in place. Nowadays, the limit is 8192 processors according to arch/x86/Kconfig (see also: https://access.redhat.com/articles/rhel-limits).

It wouldn’t surprise me if Microsoft spends more time and effort on tuning and providing OS services for the particular commercial applications that are typically used on high core-count Windows server boxes, several of the most prominent of which are also Microsoft products.

I suspect SQL Server has no trouble scaling to 64 cores and beyond.

That is true. I have seen some terrible queries written on enterprise hardware that returned at the drop of a hat without being cached ahead of time.

I suspect it's just simple math. In the same way that Windows deployments vastly outnumber mainframes, (large scale server) Linux deployments vastly outnumber Windows. FAANG (and want-to-be FAANG) companies have been hammering away at Linux for decades with workloads that have grown to scales Windows has never seen. For example, how many billion user web services are running on Windows? What percentage of supercomputers today run Windows? etc.

Well the Windows Kernel is known to be well engineered. The userland including user interface is a different story. Here the backward comparability and the huge feature sets kick in including all its disadvantages (and advantages)

I have often wondered if Microsoft will one day Open Source the Window Kernel.

My guess is Linux is still mainly used in server environment so utilizing more threads as possible is necessity so more contribution is going into this ,