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

Depends on the country where the exchanger is running it's operation.
See those exchangers that you have listed all run in the US, where bitcoin is officially considered a legal currency and laws apply to it which is why they are required to have license.