Cryptography-related code is incredibly easy to fuck up, and the tiniest mistake can completely break your security model. A mistake that could be insignificant

Cryptography-related code is incredibly easy to fuck up, and the tiniest mistake can completely break your security model. A mistake that could be insignificant in other kinds of code, could easily be critical in crypto-related code. Thus, you reduce both the amount of mistakes and the impact of mistakes by avoiding crypto where possible. EDIT: Yes, this includes using existing libraries. It's not just the implementation of the primitives themselves where issues occur.