Why is bestest not a word?

Answer 1

Because the word "best" is already a superlative. The word "best" means "better than all the rest." There is nothing better than the best.

For example, if I think the Wizard of Oz is the best movie ever, it means I think there is no better movie on earth than the Wizard of Oz.

If I said that the Wizard of Oz is the bestest movie ever, it would mean the exact same thing: that I think there is no better movie on earth than the Wizard of Oz.

Since the word "bestest" would mean the exact same thing as "best," there is no need for such a word. It's redundant.