They do that to keep you playing longer. I wonder if OP would describe the loot opening dance in games like hearthstone and Overwatch as rewarding game feel, rather than an intentional time sink.

Remember, for a user centric app, the goal is for the user to use it AS LITTLE AS POSSIBLE. Not because it is frustrating to use but because it is efficient and you respect their time. Optimising for stickiness is user hostile.