Probably because most of the Apple OS updates are more or less equivalent to Service Pack Releases. Remember, Apple has yet to go to OS 11, all the updates have been incremental!
Mind also that Apple is now pushing those updates to once per year, it was previous once every 2 years. So you can wind up paying more over the same period of time as it takes for MS to produce a new OS version once every couple of years or even longer.
While this is at least lower than what they were charging for Windows 7 OEM when it first came out, never mind the retail box pricing or the fact you had to choose between a larger number of versions...