When you use Windows, you don't use the alternatives, don't consider the alternatives, and don't change your habits, which makes one even more inclined to stick to Windows. That's why Microsoft is careful to not make it too hard to acquire a copy of Windows. (On the other hand, it makes it hard for hardware makers to offer anything else, and for businesses to switch or elude.) A regular user doesn't care about the OS anyway, and would follow any shepherd. As a wrapper to run browser and multimedia player, Linux was ready for desktop long ago — see famous anti-Linux stunts at Microsoft.
Moreover, the leaders today don't profit from software, they profit from data collection and control over their “free” services. The bigger the user numbers, the more money you can get, and when Windows activation can be done by typing some commands on a vanilla system, it is effectively “free” for everyone who wants to take it.