They release the OS in September (or very early in October), leaving only 3 months of the year where the date is wrong. Doing it the way you're saying would make it wrong for 9 months of the year.
The only way to avoid the problem would be to release the OS' every January 1st. Or they could pull a Microsoft and keep the release year in the name for several years so the marketing team can take an extended multi-year sabbatical. 😆