The last time I bought a windows os was a floppy disk version of 95. Since then every computer I get has a different windows with it. For free.
No, your system builder paid the "microsoft tax" to be allowed to distribute their system with windows. And the best thing is that (until not too long ago) you would EVEN pay this fee if you happened to buy a system *without* windows from any largish place, since not paying for the OEM license for *every* computer they shipped would violate their licencing agreement with Microsoft resulting in significantly higher rates to distribute windows with a new PC.
Must be nice to have a monopoly, and even get money when people DON'T use your products.
Interestingly enough, software like your 'free' copy of windows makes up a disproportionally large amount of the purchase price of a cheap system. Hardware components have come down in price tremendously over the years, but software prices only creep upwards.