Not all the casinos will definitely have a similar interface, but you may discover a number of them have one or two things that could be linking to each other, operating under a similar gambling platform user interface does not really interpret for mimicking such a platform as long as we can also put in some vital features that are going to create a visible difference from what we offer to what the also have on their platform.
You are right, and I say that no matter how similar two or more casinos look in terms of design and all, there definitely will be lot of features that seperates the casinos.
And this is because it's only a team that is not serious about building their own brand and signature that will outrightly copy both design and features of a casino into their own casino, in the end, this can also be very bad for their business because most gamblers will see the casino and as a phisher and potential scam casino even if the team behind the casino had some really good intentions.
I know that a lot of casinos already exists that is hard to come up with a design that isn't already in use, but I believe that design is like music, there will always be room for new and unique ones, all it takes is getting a really talented person to handle the design.