I guess I'm not sure if you're suggesting otherwise, but gamification is not unique to interviews. It's a natural response to any system that relies on metrics or measurements. The request to design a theoretical system is gamification as much the guide for answering it is. These games exist because it elicits answers that can be used to learn about someone's real experience level.

It's great that you feel confident relying on your real world application but it takes a long time to get repetition on building things at scale in production. Practicing system design is a way to shorten that learning loop.