Because they were essentially created because of scientists' discomfort with the way that various elementary field strengths and thigs like the mass of the electron seemed geared towards the creation of "stuff", including life. So, they developed this theory that there are an infinite number of universes, with the settings on field strengths and masses at various levels. That way, they can say there is nothing special about our universe in particular.
But the intriguing thing to me is that if there are really an infinity of universes, where literally anything and everything can and will happen, then there is a universe where a God created everything. There must be if there are infinite possibilities, right?
This would seem to defeat the whole purpose of proposing the multiverse in the first place.