They can't add sneaky code to the process (without getting caught). For sensitive game code every single change needs to be tracked and reviewed by the authority. You get audited at least once a year, and then all the changes are reviewed. Authorities outsource the job for the technical reviews to specialized companies.

Also, what's the point? The games already provide a margin to the host, why risking to go out of business for such an irrelevant gain (a few more %)? Add to this that usually casino games writers do just that, write games and sell those to N casinos. So the incentive for the casino games writers are even smaller.

Finally, yes you can write "license X", but you can cross-check that information from the regulator itself, you don't need to trust just the line on the site. The point is you as a customer can choose a trustworthy site, ideally one who is licensed in countries where regulations are quite tight (in Europe I would say Denmark), before putting your money somewhere.

At some point you need to trust "someone", that's how the whole world works. The gambling authorities are no different than the authorities that enforce the safety certifications for electrict equipment, or cars, or whatever.

If your concern is that you would lose money on casino games because the site rigged it, it's a relatively silly concern. You will lose because the casino games are designed to make you lose in the long term, on average.