If you have fun isn't that the point. Ideally games should be a here are the simplified rules so you can have fun fast. Then here are the full rules so you can play a much more complex and fun game. Of course pulling that off is hard.
You're not going to learn a full complex ruleset in one sitting.Having a one page quick start of the game structure, allows you to get a feel for it before coming back for a deep dive.
Your brain has a foundation to build on when you read the ful complex rules - it wont feel like your brain is maxing out at 100%.
This is addressed in TFA. In many cases, attempts to create a simplified version of a game just teach bad habits; strategies that work in the simplified game might be not just suboptimal when the full set of options is available, but actively counterproductive.
But I dare say many fans of "heavier" games - especially ones with more of a simulationist bent - would disagree that having fun is sufficient.