Because there are always someone who think they learnt enough and do their own implementation, putting the data of them and others at risk. Writing secure implementation of crypto algorithms is hard and there are millions of ways that you can screw up.
A lot of things are hard and yet people do them and implement them all the time, that it might be hard is not a reason for people to not implement things - if anything by implementing them is how they'll learn (especially for those, as mentioned elsewhere in this thread, who learn better by doing).
You can screw up all sorts of things - which may (depending on the situation, application, etc) also end up with data at risk (assuming that there is even such a requirement in the first place). That also doesn't mean people shouldn't implement crypto, otherwise they shouldn't implement anything that touches any data at all - after all someone might use the most secure library in the world and accidentally forget to check for user authentication (or whatever else that has nothing to do with crypto itself but still check for privileged data access) in a specific way and let the data leak out.
And honestly i'm personally vehemently against this elitism-preserving "this is for us enlightened few to dabble with, not you plebeian hands" attitude towards crypto that some people have as if it is some forbidden knowledge that only a few high priests can have.
Don’t selfhost. You inevitably will misconfigure your servers and lose your customer data. Bow to amazon overlords and don’t even think otherwise.
Don’t drive. You inevitably will kill some pedestrian. Use Uber and let professionals do it.
I don’t think that’s a healthy attitude. And exposing data because of misconfigured firewall (ahem, DOCKER-USER, ahem) is like 1000x more probable than someone hacking your cryptoschemes.
There must be freedom in software engineering. Even at cost of some grave mistakes.
