I would even go a step further and say that writing the software from scratch will most likely result in the casino not surviving for long:
In addition to the very high costs (development team, testers, ...) for development and, above all, maintenance, it must be assumed that the software will not function smoothly, at least at the beginning. On the one hand, this naturally concerns bugs that annoy customers, but it can also lead to customers exploiting errors in the software and the casino losing a lot of money. There have been more than enough examples in the recent past where casinos have lost thousands of dollars due to such bugs.
That explains the existence and survival of game provider companies and many casinos use games from third party game providers, big and famous ones. They can save a lot of cost from product development, test, security, maintenance and more. By partnering with game provider companies, casinos can quickly have available products that already fully developed, tested and they can avoid risk from bugs, exploitation too.
Develop your own built games sounds good and impressive but how it will work practically is more important for your business.