In some cases, there is risk to doing it yourself (rolling your own authentication scheme). But from a pure learning standpoint, yes people should be encouraged to build even if solutions exist. Perhaps there is a better version we don't know about until it's built.
When it comes to a startup or company project though, it also depends on how much time you spend on something that isn't the core problem.
I would add that doing things on your own, which is probably never optimal from time perspective, may open doors to solutions that you haven't seen before.
I recently experienced this is by not going through the beaten path - installing well-known dependencies and building solution based on that. I chose to experiment a bit and it turned out that I usually use small % of imported code.
When it comes to a startup or company project though, it also depends on how much time you spend on something that isn't the core problem.