Think it's because certain topics, such as dynamic programming or graph algorithms, are just not something you can attack from first principles for most mortal people. I certainly wouldn't have invented binary trees. So there is quite a bit of things to read up on (though it should all be covered in an algorithms course).

Then once you do get that context... like LeetCode problems often aren't very inspiring? A lot of the time it's "just apply this technique here".

Whereas in AoC, at least in the first few weeks, it's mostly just, do the task, attack it from first principles.