I should have specified that I'm talking about EU-based neobanks, which are all real banks as far as I am aware (some are entities of bigger banking groups, some are standalone startups).

US banking being largely obsolete as a whole (checks? paid wire transfers that take days to arrive?) probably explains why there are no real neobanks in the US and they're all just a UI in front of a legacy bank. I doubt it has much to do with regulations, because I really doubt US banks are more heavily regulated than EU banks, who also have country differences to deal with if they operate in multiple countries (Revolut, N26), and they also have to check if the customers are American tax residents and handle that case too (or simply refuse them to become customers as some banks do).