Well, the new gambling sites tend to copy the old and trusted gambling sites and therefore they even try to copy the user interface too. The positive reason can be that they know that people are comfortable playing using such an interface so why not keep their site with a similar user interface? However making a complete copy to copy site, will never give a good impression as people will think that it may be a fake or scam site.
Other than this, the scammers also get the same themes and apply them on their sites so that they do not have to develop new designs as their main purpose is to bring people on the site and scam them. Also, people who want to develop a phishing site would also develop the same copy of the website.