Genus isnt sexus. The chair isnt female in Spanish because ancient Spaniard thought it might have titties, its feminine because of its phonology. La silla ends with an a, so it gets the la article.

Why does English still use actor and actress? why do you use definite articles instead of marking definiteness with prefixes? Why do i have to "i saw Mike steal your bike" instead of included something inside of the verb "see" to mark that i witnessed it?

Language is just that. An evolution of changes that happened throughout millenia and rarely was any pressure applird to that change.