Because there aren't so many great alternatives.
There's MacOS, which requires expensive dongles (an Apple computer), and has some of its own systemic issues.
And there's Linux, in different subtly incompatible distros, with its own share of problems, and a non-starter if you need and/or want to use many proprietary staples, from the Creative Suite to video, music, and business apps...
Windows doesn't require a computer?
As in - if you already have a windows PC and someone says "you should try a Mac" you can't just install MacOS, it requires you to buy a whole new PC from Apple. With Linux you can at least just install it in a second partition and use it that way untill you're sure it works for you, zero money required.
The point they were making is that a Windows user generally (Hackintoshing aside) can't just install MacOS in their existing computer; it requires purchasing another, Apple-brand computer.
One of a generic PC-compatible machine, available in all kinds of prices, with all kinds of specs, from all kinds of vendors - one of which you likely already own anyway.