Opening a casino is a different thing and getting a suitable license is another thing so we don't need to confuse ourselves with this.
Really you are right, someone that thinks of establishing casino game supposed to have plans for the license. What op is trying to sure of, is if casino can be established without license, and answer is (yes)
There are many casinos that are never licensed at all and they are still in existence till now. It is more beneficial to the casino because it will be hard for them to be sued because they have no license at all. Most if these casinos mostly end been a scam because there is no regulatory bodies that is making sure that they are fair and suitable in the business they deliver to the general public.
There are many scam casinos and gambling sites in the market which there motive is to scam customers of their funds.
Why people emphasis on license is for the safety of the casino platform, those ones that have being operating without license, it's clear that they are operating illegally and anything can happen to them, because if you notice them and sue them, the surviving of the platform with any allegations alleged to them will be very hard