I may be of a different age, but in my time it was common to buy computers without any OS pre-installed. It was on you to install one (from CD, floppy, whatever).
Of course, vendors selling PCs often also sold OS's...
But I also believe your recollection is erroneous or must differ by region because I was there as early as the 80s and computers sold without an operating system weren't common. If you were buying a PC clone in the 90s you could buy it without the Windows license, of course, but the default was to include it.
Even before PCs, computers typically shipped with BASIC, which was their operating system of sort. Macs, Amigas, they all shipped with an OS as well.
Sure. In the 80's, all the IBM PC compatibles I dealt with shipped with BASIC in their ROM. But I don't think they charged a licensing fee for it.
I don't think Compaq even made XTs, did they?
Here is an HP without an OS: https://www.newegg.com/p/1VK-001E-4UPC7
If you want something that most other people don't want, it is going to be a little harder to find. That isn't a conspiracy, that is just life.
I'm sure they did sell computers with Windows pre-installed in the 90's. But almost every place also sold computers without any OS. It was not at all unusual.
The computing world wasn't just in the US, you know. Microsoft didn't have a deal with all PC manufacturers in the world.