Takah, it must be because Matzah is round. And therefore, similar to what Popa said, if you eat a slice you’d have to wash but the pie is round so that can’t be Hamotzi!
Well, no. The thing with Matzah is that it is very thin, like a cracker. Even there, when you are obviously Kove’a Seuda you would have to wash.
The Jewish fast-food fast-Bracha industry will have to find another solution. Now we can start making cooked pizza.