Very few casinos this days (and by this days, I mean casinos that was launched in recent years) have softwares built from scratch, and even at that, it can never be all of their softwares.
Like you rightly said, building a software from scratch involves a lot of work,  which might take years due to how much time that is needed to test and correct every error before releasing the software to the public for use, hurrying to release a half baked software can be very detrimental to the business, as hackers can easily identify an error and use it to hack the platform.

So, instead of taking such risk, which is also time wasting as well, most casinos today simply go for the same software that other casinos are using, this ones have been tested and trusted to work, the casino only need to change a few things to sort their own business model, and boom, it's running.