>I'm somewhat surprised that Windows is still most of personal computers.

It's the easiest route. On non-Apple computers, Windows is already installed. (It takes a bit of effort to buy a computer without an OS.) Microsoft makes it easy for a user to get Microsoft 365. With that, users have the computer they have at the office and are familiar with. Most are just surfing the web and writing an occasional letter, anyway. That doesn't include people who are perfectly fine with a Chromebook or just their phone.

Finding a Linux distribution, downloading it, putting it on a USB stick (or burning a disc), then installing it is not simple for most people. (Don't even ask them to verify the checksum.)