More of what I think is the same issue/rule:
Ich habe ein gelbes Buch. Why can't I say "Ich habe ein gelb Buch."?
Maybe the answer it just because. I'm really enjoying working on another language but do we really need masculine and feminine version of words. I know German isn't as bad as the latin languages with that but it makes vocab so much harder. And add das der and die on top of it