The OS requires minimum hardware. To force users to upgrade their OS, discontinue the old OS, and make a new OS version, which has greater minimum hardware requirements. Now the user is buying your software again.
They're also buying new hardware which benefits the PC maker. It's a mutually beneficial relationship that forces the user to both buy the software again, and buy new hardware. (You do pay for Windows when you buy a PC, it's a cost the manufacturer absorbs. You can often receive a discount when you order a new PC by not including Windows with it.)
From my experience it's actually the opposite. The PC is sold with Windows on it, purchased by the OEM. The OEM then loads crapware on the new PC before delivery because crapware companies pay the OEM to load crapware. As a result, it'd actually cost more to buy the device without Windows.
I've only ever seen one piece of x86 hardware that was sold with or without Windows in my lifetime. It was $15 cheaper at the time to buy the Windows version and install Ubuntu myself.
They're also buying new hardware which benefits the PC maker. It's a mutually beneficial relationship that forces the user to both buy the software again, and buy new hardware. (You do pay for Windows when you buy a PC, it's a cost the manufacturer absorbs. You can often receive a discount when you order a new PC by not including Windows with it.)