What happened to peep?

This is going to be a short entry, but it's food for thought none the less. I'm not really sure why my brain thinks what it does sometimes, but I can't argue with it. Let's just get to it, shall we?

There's POO and PEE. There's also POOPOO and PEEPEE. So why is it that there's only POOP and never PEEP?

Answer me that!