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.
BTC is not considered a legal currency in any country. It is considered a commodity or asset. Also, the same laws operate in almost all the countries. Once you are running an exchange/broker/forex business (accepting fiat or sending fiat) you must be licensed.
USA is enforcing the laws much better than other countries, that's why some exchangers registered in US have applied for financial licenses. The rest of them are operating illegally.
Bitcoin seems to be the worst magnetic attractant for criminals since the prohibition era liquor business or the 40s-50s Vegas gambling business. Too bad Bitcoin didn't exist in the mid 20th century. The Chicago, New York and Vegas La Cosa Nostra would have loved it. LOL