Because when letters fit, they get to continue to live and reproduce more monkeys that also already know those letters :o) Monkeys that don't, they die out along with their bad spelling and faulty spelling genes. Soon we have lots of monkeys that spell very well. The "fit" therefore is analogous to those traits in monkeys that allow them a competitive edge in the game, whether the game of survival or the game of "Let's write all of Shakespeare's Plays". The "play" is being written every single day and "all the world's a stage", to quote some monkey.