The universe is compromised of all that is. What is not in the universe? Nothing.
People have trouble conceptualizing nothing. We tend to think of empty space when we think of nothing. But if there is truly nothing, then there is no space either. Complete nothingness cannot be experienced.
Then various people want to go on about the nature of reality, and they don't really understand the meaning of the word universe, and they come up with the term multiverse, and, well, it can't be so. This is not to say whatever they are struggling to say is not true. A multiverse can't be so linguistically- that is, the language is defeating them. If whatever it is that they are referring to when they say 'multiverse' exists, then it exists within the universe.
Whatever is is inside of the universe, so calling a class of existing things multiverses seems likely to cause misunderstanding.
This is highly similar to the reality of now. What is, is now. The past and future are not, and do not have the quality of existence. Nor do they have the quality of space, which 'time travel' movies indicate we are inclined to imagine. The things that will be are not yet. And the things that were can only be said to exist if we check the now and see they are indeed still here.
I see two sources, or two temptations that lead us to get these things wrong. The first is uncertainty. Lending the past and future the luster of existence helps us feel less uncertain. The second is the modern anti-metaphysical bent, blurring reality a bit helps one to avoid disorienting questions.