Why do people exclude the possibility of more than one big bang?

The possibility of other universes from other big bangs is acttually widely considered feasible by string theorists. One interpretaion is that our's is just one of a "landscape" of universes in which only the tiniest fraction are able to evolve life capable of being astonished by how "lucky" we are to be living in a universe where properties are so precisely within margins necessary for such life. The catch, though, is that other universes are causally decoupled from ours since they all have their own disjoint spacetimes. That is, they cannot be observed and, therefore, one cannot form a valid hypothesis about them in the context of the scientific method.