I don't think we talk enough about the downsides of DRY.

Reinventing the wheel for the sake of reinventing the wheel (not-invented-here) is a problem, but reinventing it for the sake of learning more about wheels is a big deal.

I suppose many developers reach for libraries because they're more confident that the libraries will implement things correctly/more efficiently than they could. But if they keep reaching for libraries (instead of trying to write an `isEven` function themselves) then they never really improve either.