First mover effects, network effects, brand recognition, corruption, rent-seeking over the assets created by the aforementioned

Like, yes the main point of leverage obviously isn't software quality, but also, if AI can't cause one person to be able to solve those other problems at some scale that's currently not possible with no other human help, those problems become problems someone else could also throw the AI at. If running a business relies on having some point of market leverage over anyone who pays your business for something, the reduction of the cost of independently gaining that leverage necessarily means the reduction of the value of your business

I guess another possible means to a single-person company with a billion dollar valuation is hyperinflation