I know I would not be able to focus for 4 hours on a test that doesn’t matter, while reading text I don‘t care about, so that kids are failing that seems like a non-issue to me.

The rest if your point is simply wrong. Focusing is a physical activity that you can train for. Every good chess player that plays classical trains for that. And so do (or rather so did) students in the education system. Taking mock tests before the finals is a way to train your endurance such that you will be better able to keep your focus throughout the test.