Because you have enough to try and comprehend and retain. This is what the employer training period is for. However, I get shouted down and downvoted when I mention that 3 weeks of training is not enough time. I get that people want to stop training and start earning, but it takes time to come across all the different situations and scenarios to learn what's needed to know.

I trained new drivers on flatbed. Sometimes I needed to beg dispatch to find coil loads so they could get the experience with a trainer rather than figuring it out on their own. You can only learn so much by looking at diagrams, you need real world hands on experience.

So spending 5 minutes in a class learning that you can have more than 12,000 on your steer axle is likely to be promptly forgotten since there's no questions about it on the test. But coming across this situation in real life during your training and going through the motions of looking up the limit in different states and verifying your equipment's rating will actually be retained in your memory.