There are two major things going on.

First and most weird, many games don't focus on advertising themselves. Instead, they pay online marketing companies to find them customers. Those companies then produce videos, put them on various online services, and try to get people to sign up for the game.

This provides an odd incentive problem. The marketer's goal is to get you to install and play the game long enough to get it to count as a successful customer acquisition. They are not in the business of getting players to stick with the game. Because of that, they may feel free to completely lie about the contents of the game, so long as it gets sales.

The second issue, though, is why some game companies lie about what's in their own games themselves. It turns out that online game marketing is a big numbers game. You show a million ads, 0.01% of people click on the ad, you adjust the ad, 0.02% click on the ad, you just doubled your ad's effectiveness. Wow! Because of this, there are a lot of marketing companies and standard practices out there, and it turns out that making good ads is hard. So, instead, everybody makes the ads that they know work. For example, the "watch somebody pull keys wrong so the lava falls on the treasure" ad format is pretty effective, so everybody started using it, despite what their actual game was about. If you like the game, it doesn't matter that the ad didn't match up, and if you don't like the game, it also doesn't matter. Mobile companies actively discuss the pros and cons of these misleading ad strategies, not as an ethical issue, but just as another strategy in a numbers game: https://www.mobileaction.co/blog/user-acquisition/gardenscapes-ad-strategy/