It is the default. Almost all computers today come with Windows. If someone clearly unknowledgeavle were to ask the salespeople in any PC store, they’d say some variation of “just use Windows”.
Microsoft managed to make sure “a PC” almost exclusively means “A computer [with Windows]”.
You missed reason 0:
Microsoft managed to make sure “a PC” almost exclusively means “A computer [with Windows]”.
And they have spent a lot of effort for litterally decades to make sure most machines are as difficult as possible to use with anything but Windows.