Mostly because everyone uses tags differently, and forcing inheritance would be a lot less flexible than what we have now.
I can see cases where I would want all of an album’s tracks to inherit its tags (or all of an artist’s tracks/albums to inherit the artist’s tags), and I can see cases for the opposite. I’ve been thinking about a number of ways our tagging functionality could be a lot more interesting and/or dynamic, mostly spurred by @AndersVinberg’s post here.
Give it a read, and let me know a little more about what you’re looking for – how would tag inheritance meet your needs here? What are the cases in which the current system falls short?