You seem to be confused.

A person can have an ownership share of $1B and not own the entire company if the entire company is worth more than $1B. For instance, if you own 1% of a company that's worth $100b, the company isn't "in the hands of a single person".

But even still you're saying that one person cannot create a $1B company which would mean a lot fewer $1B get created. Which under "big company bad" mentality, sure maybe it's a good thing. But that's obviously an insane position to take