Because they don`t have choice.Bitcoin is still illegal in some countries.
I don`t know is it possible for a cryptocurrency exchange platform to get a license.
see Coinbase, Gemini, ItBit, Circle. all these got licenses, so it's possible if you really want to run a legit business

that's right.
and also add to that being in US, following the US laws such as keeping logs, paying taxes, asking for all kinds of documents for verification purposes, and many more.