All agree that speculations, even if they come from mathematics that seems to make sense, still need evidence in order to be believed. And a lot of the speculations about multiple “universes” seem to be beyond observational evidence, at least for now.
But it seems to me that some of the puzzlement comes from calling these hypothetical entities multiple “universes,” of which “ours” is also a “universe.”
What is a universe? If there can be multiple galaxies then why can’t there be multiple entities that are bigger than galaxies and include galaxies? Let’s call them “sub-universes,” and let’s say that (hypothetically) they may resemble one another in various ways, but be “out of touch” (out of observational reach) of one another. That makes them more like some of the unobservable microcomponents (like strings and unbound quarks) that are much less far-fetched than the notion of there being more than one “universe.”
(That said, I think the multi-sub-universe consisting of all the possible histories since the Big-Bang is too far-fetched to take seriously no matter what we call it. — I also think the notion of multi-sub-universes does not really give us any insight into either the probability or the “inevitability” of life.)
https://youtube.com/watch?v=rjphfKI661k