Because they say so. There is physics which deals with the real observable universe. Then there’s the mathematical physics which is sometimes applicable and useful and sometimes just pointless rabbit-hole exercises.
Yes indeed...
This is such a case...
I'm going to tune my radio. with the appropriate background noise filter, and listen to what the poor dead guy in the wheelchair has to say about this...
Then I'll decide whether this is just the latest "string theory", "super-string theory", the multiverse, or black holes as holographic entities...