Because when you learn alone you don't know what to learn. You usually only learn what you need now.

I would not say "massive holes", as that would somewhat imply that this knowledge would be actually required, even though it's usually the optional, nice-to-have knowledge that you are getting from an actual CS degree.

You can still create whatever you imagine even if you are self-taught, but going through university would make it a lot more likely that you understand better the tools that you are using (algorithms, microprocessors, logic gates, electricity, quantum effects, etc.) so you will better know how to use them and what their limitations are.