Put simply, we know of only one universe: this one. In addition, from what we can tell, this one wasn't always as it is now. It was once compressed into a very tiny space, then expanded outward. It would stand to reason that other universes developed in a similar manner. Ergo, for black holes to contain universes, it would be logical to assume that they would need to expand outward, much like our own.
How do you know they don't, though? When you say "it would be logical to assume" how a potential Universe would appear to behave when looked at from the outside
, based on observations of how a Universe appears to behave when looked at from the inside
, you've made a staggering leap. Such a proposition may seem
right, and may even have enough of a smidgeon of inductive reasoning behind it to trick our monkey-brains into deeming it acceptable; but actually, there's not really any logic
behind it. You've not actually developed a sound logical argument based on a series of correct premises. It's at best questionable inductive reasoning, at worst an intuitive leap; and in physics, intuitive leaps are fraught with danger in realms that are often counter-intuitive. For all you know, those two frames of reference are nothing alike: who knows what concepts of space and/or time may apply to such Universes? For all you know, there's some entity discussing with some other entity that the black hole they can observe appears to be doing the square root of F.A., and how it cannot possibly contain a Universe - all the while unknowing (and us equally unknowing) that they're staring directly at us.
Now, that's not to say that you're wrong. For all I know, you may be right. But it's a bad area for making assumptions - no matter how "logical" it may seem to make them.
I stand by my assertion, based on the fact that everything we can observe is limited.
That's basically an argument from ignorance, though: "Everything we can (currently) observe is limited, therefore no actual infinities can exist at all in reality."