I've taken junior level college classes that relied on concepts taught from freshman year without any class in-between reinforcing that knowledge. Hell, I've had college courses continue concepts that were last taught to me in middle school.

I feel like part of the problem is that students demand full understanding as to why they need to know a specific something immediately while having little context as to why this may be important. There are also cases where it may be important to some students in the class, but the school doesn't know which students yet because those students aren't there.