Even though Linux is still somewhat popular in tech circles, consider that windows would have a significant market share for providing high value entertainment and a wide variety of tools to office workers. Microsoft Office is the dominant documentation and accounting suite for office workers around the world.
Now, combine that with the way that Microsoft has bundled their OS into many laptops and retail computers worldwide and you see why they’re big.
Essentially anybody looking to do any paperwork related work will have to interact with Microsoft’s system of software in one way or another. If Bill Gates was a deity, he’d probably fit right in with the god of tax collectors, taxing people for paperwork and bureaucracy.