Because human mind concept of vastness is limited by the senses and the things that they could see, hear, smell, touch or to be more accurate, the things that their senses could detect.
Like one could not understand the concept of a planet that is one hundred times larger than Earth with their minds.
Of course, when one gives a visual representation, there is a concept of this vastness. But one still could not truly understand it because it is hard to imagine such largeness.
As such, the largeness of the Multiverse is even harder for a human mind to wrap around unless one reach a certain level.
There is vast array of Universes
There are universes, part of a multiverse, of more than ten to the power of ten to the power of sixteen distinguishable realities.
That is the most conservative estimate of all kinds of Universe that existed in the Multiverse by scientist before.
