Like anything in life, context is everything.

This "Don't reinvent the wheel" thing comes from the observation some people (software engineers in this case ) tend to spend a huge amount of time ( aka money ) creating things that were already created AND could just be used by them. It's a very valid point, especially when you have to pay salaries every month and don't have a money printing machine.

However.. the motto is also used to justify mental laziness and protect egos from ignorance.

People learn by doing, you can read all the books you want and you'll be a very competent parrot but you'll never really know. The question is, what do you choose to know and what to you choose to parrot? Since time is finite (for you ).