Why Don't Every "A" And "B" Become "AB"?

Just had a random thought in my mind when a song sung by Paul McCartney and Stevie Wonder crossed my mind.

Ebony and Ivory
Live together in perfect harmony
Side by side on my piano, keyboard,
Oh Lord, why don't we?

*written by Paul McCartney

So, why can't we just live together and give each other, thus we will survive our life?