Why cant we just buy the OEM version and install them ourselves?
Can or not? The retail version is damn expensive meh.

Once installed, the OS is 'tied' to the machine.

How do they define a new machine?
(swapping CPUs? mobo? HDD?)

What if the primary HDD got corrupted (and
we're unable to properly uninstall the OS)
which we would then need to reinstall the OS
on a new HDD (on the same machine) ...