Musicians aren't bad at math - they're just playing by different rules! In music notation, 6/8 time isn't a fraction waiting to be simplified to 3/4. It tells you there are 6 eighth notes per measure, creating a completely different rhythmic feel than 3/4 time (which has 3 quarter notes per measure).
It's like saying "why don't we just call all dogs 'small wolves'?" Sure, mathematically they might look equivalent, but try telling that to a conductor when you're playing a waltz instead of a jig! The beauty of music is that these "unsimplified fractions" create entirely different emotional experiences. Math and art collide in the most delightful way!