And people know that the US is friendlier to startups/business than other countries, so founders will come to the US. They didn't just happen to be here.

Does the "I can always go back whence I came" also lower the perceived risk? If a domestically-born founder fails they aren't apt to have anywhere else they can go, thus having to stick around to deal with the fallout.

I can imagine the opposite "This place is so nice and I don't want to go back to what I perceive as a hellhole", and so they work harder to succeed.

A significant number of these founders are from Canada. It is not that much different from the USA. Canada is less business friendly, perhaps, but that isn't important after you've failed.

Ah yeah, if lazy Americans would just try harder, they’d be richer.

tons of studies show children of bottom income quintile immigrants have far more economic mobility than non-immigrants children in the bottom quintile, so there is something to it, even if it is not as simple as laziness.