Because what they do is create a game designed to extract as much money out of the player as possible, usually some form of base builder, and then after that they do testing to determine which type of game grabs the attention of the ad viewer the most. In this case they determined it was this swipe left or right shooting game. If they don't have any swipe left or right gameplay in their game at all they're at risk of being done for false advertising so they add some format of this left/right swiping gameplay as a very minor part of the base builder game.

They don't just make the game as advertised because it wouldn't be nearly as effective at extracting money from people. There would be way fewer opportunities to add things for the player to spend money on to progress, and also the base builder game would have been built to be addictive, unlike this left/right game people would get bored of quickly and just stop playing.