The billion people you mentioned use Windows because it was preinstalled on their computer. They also have not, in their majority, heard of Linux. There is almost no company doing marketing for Linux desktops.

Many people use computers for entertainment and Windows has more compatibility with games because game developers target Windows. Even though many of the game engines they use support Linux.

With Wine, Proton, Dxvk, etc... the game compatibility gap is closing rapidly, though.

Now, many occupations require software that won't run on Linux (software from Autodesk, Adobe, etc), but the use case for the vast majority of people is well covered.

If they were given the chance to acquire their computer for cheaper without an OS they would have done so.