There's a few things that are relevant:
- When you're an adult, you know more of the context. Everyone as a kid has at some point thought "why am I learning this?". When you're older and you've seen a bunch of stuff, you know why.
- When you're older, you are typically not under pressure to pass an exam. IMO exams cause horrible problems. You're often under pressure to learn more than one class. And you more than often need to learn a lot of specific things that you need for the exam, but are not required to understand the subject. Like what the exact form of some equation is that you for some reason aren't allowed to look up in the exam. You end up spending a lot of time as a kid learning how to pass the exams rather than learning what you're supposed to learn.
- Adults have a better sense of when they actually understand something, rather than just superficially. Part of this is the classroom tyranny. As a kid, you are quite conscious of being judged. After all your future is on the line, and if the teacher thinks you're dumb, bad things might happen. As an adult, you're in class because you want to be in class. If you don't understand something, your status as an adult also helps in how help is presented. You don't get the "omg why am I teaching this kid chemistry, I could be making meth for big bucks".
- When you're an adult, nobody thinks to limit your quest for knowledge. If a kid asks "why is light a particle and a wave" you give them the Discovery Channel answer and stop them from asking too much. If an adult asks, they are welcome to sit in a quantum physics lesson until they leave by graduating or quitting.