You have proposed an absolutely correct method for assessing the honesty of a casino. But it is not profitable for them, because an open source project will be immediately copied. And casinos spend hundreds of dollars on developing their code, on which the mechanics of gambling will work. But as a gambler, I would show a lot of trust in those casinos that could prove to me through blockchain that the result of gambling is honest. But so far I have not met such casinos.
The problem is not with the casino as their games developed “house games” are all open source rather it’s due to the game provider that wants to protect their properties from copyright like what you mention and also to avoid exploits since no one can see their code except.
Should online casino games be open source? The simple answer is yes, Because if the casino is open source there are lot benefit of it. According to google it includes cost-effectiveness due to being freely available, increased flexibility to customize and modify the code, enhanced security through community-driven code review, transparency as the source code is accessible, reduced vendor lock-in, and a collaborative community providing support and innovation
See I highlighted some of the word transparent and security the biggest important thing on casino
It’s true that being open source makes the code stronger but this is casino we are talking which means there’s a lot that will choose to exploit rather than build with it.