Well, perhaps because many things which could not be explained in the past caused people to think of these things.
Sorry if this doesn't help, I've just started learning these things in school!
It depends how you define "famous." Furthermore, it is hard to define what makes up a full myth. Someone might say that everything pertaining to the Trojan War amounts to one myth, while someone else...
There are many Greek myths. There is so many that it's hard to count them all. They have been altered and passed down for many years making it hard to count. If this doesn't help.... ASK GOOGLE!