Remember also that operating systems, are, well, systems.
You buy "the whole enchilada" to get
"the whole enchilada."
It's just like a car in the sense that "you don't 'buy' the transmission ... even though you can't go anywhere without one."
Microsoft, in particular, sells a very tightly-integrated suite of software products which run on their Windows OS and, with a very few exceptions, nothing else. In doing this, they're free to put the components anywhere they need to be including the kernel itself. They can, and do, integrate the products very seamlessly into the user-interface shell and so forth. And this
is what so many businesses have purchased: not just "the Windows OS," but "a business software suite for which Windows is a means to an end." It's not a bad strategy, and it has earned them a lot of money. Their only drawback is that Microsoft has no control over the hardware. They licensed their system, as Steve Jobs once put it, "promiscuously."
Apple took a hardware-oriented approach that is even more "tight." But they don't have any particular suite of software to go with it. Instead, they strictly control (license ...) what software is and is not allowed to
go with it. The underlying OS is Darwin (open-source Unix), but what matters is everything else that's on top.
Linux, of the three, is very different. It's an "enabling technology," really. It runs on over 25 very-different hardware platforms and it is full of options. You can run a mainframe with it, and you can run an air conditioner. Or, as "Android," millions
of phones, tablets, music players and so-forth.
So, if you talk "popularity" but assume
"PC-class machine," then you've completely missed the most important point. Microsoft, for example, has used the "Windows" brand-name
to describe several very-different operating systems, one for each type of hardware platform, because they don't have
"one Ring to rule them all." Whereas Linux is
that "one Ring," and that
is the primary reason why it exists and why it was built this way.
They're all three "different," and they're all three equally "valid" and quite $u¢¢e$$ful.