It's difficult to describe just how many people are using Windows not because they choose to, but because they have to. Whether it is because the corporations they work at only give them Windows PC's or because whatever software that they need only runs on Windows. Being able to choose your operating system that you also do work with is largely a luxury of software engineers, I think, but for your average Joe you get what you get, even if it sucks.

Microsoft has an amazing sales team forcing vendor lock-in at corporations, schools and governments all over the world, no wonder they get tons of users.

It's a luxury of market competition as well. Gamers can choose Linux thanks to Valve.