This is not a solution and no country will agree to that. Test are too expensive and if that would be applied for casino visitors then many other businesses could request the same. Besides, like you've said, there is no guarantee that results are fully correct. Self discipline of people and to obey medical recommendations is the key but that is very hard to achieve.
And what is wrong is that each person decides whether to go to him in Las Vegas or not? Whether to risk my health when visiting a casino, I can decide for myself. In the end, mass protests in the United States did not lead to an increase in the number of cases by several orders of magnitude. Therefore, it is unlikely that a visit to the casino will significantly increase the number of patients with coronavirus.