And a pretty mainstream one. One of the niceties of software is that it scales quite easily and that if it really solves an important problem, it can grow explosively all over the world, where people do have the same problem that needs solving.

Specializing in a niche corner of the market can be called a success, but a cunning dwarf is still a dwarf. For one, it lacks the necessary capital to invest into something more risky but potentially more rewarding.

A whole lot of US unicorns haven't even tried to crack markets outside the US.

That the US by itself forms an easily accessible largely single-language market is one of the reasons they do disproportionately well.

None of the listed examples were purely software. You can't build a website that store health data about patients and scale it to the whole world and its dizzying regulatory variations overnight. You can't setup a shop for second-hand electronics that serves the whole world overnight. Etc.