It's extremely easy to get cryptography wrong if you implement it yourself and don't have an extremely solid understanding of it. Of the home grown implementations I've seen in my career, I can't think of a single one that did not have catastrophic weaknesses in it that were easily exploited leading to an outright break in most cases or at least a major weakening of the protection.
Beyond that, even if you have the skill set and understanding to do your own implementation, the possibility of other weaknesses against the implementation itself is high for things like timing attacks or actual bugs in implementation that may leak information directly even if things work correctly in an ideal case. For these cases, it isn't that the implementers have a better understanding necessarily so much as a lot more people have used and tested the implementation and there are far more people looking to make sure it is secure.
If you implement yourself, you have a very small number of white hats looking at it and a potentially large number of black hats, so you are out numbered by attackers. By using a large, highly used implementation, it balances the number of white and black hat hackers attacking it to be a more even mix.
