Ehh, I always took the “don't reinvent the wheel” advice in context of “wheels” being very simple things that everyone can create but no one wants to spend time on. It's typically not really a learning exercise to say implement quicksort or some hash table; it simply takes time.

You will also rarely build a better implementation of these things than whatever is in the standard library or even some other library that already exists. If anything, it's better to, if one have a better idea, to contribute one's patches there.