Just about worst thing these absurd rules achieve is that effectively only rule breakers are allowed in.
I see the same problems in internet debates about which programming language to learn first or how to learn programming in general, and a lot of it just doesn’t take into account that learning is a long journey and staying on the path is often more important than taking the fastest or cleanest path. If trying to roll your own stuff is more engaging, great!
No, its not absurd.
Sure, I agree, for many things in life you can "learn by doing".
But this is cryptography.
There is no escaping that cryptography IS mathematics and an algorithm built on top of that mathematics.
Unfortunately the only way to learn the theory is by reading and understanding books or academic papers.
Otherwise you are just taking shortcuts based on a summary that someone else has written for you on a short blog or forum post. Which, aside from the fact that you are skipping the detail and hence not really learning, also brings us into the region of trust...
If you have no interest in learning the dry theory behind the cryptography, then you have no business rolling your own crypto. Because if you are not willing to understand the theory then you are going to make serious mistakes.
As someone who wholeheartedly buys into the “thou shalt not roll thine own crypto” mantra, this attitude is just sad. Everyone who wants to has all the business rolling their own crypto for learning purposes, and you have no business telling them they shouldn’t. Stifling this sort of exploration is fundamentally wrong.
First thing this mantra does is that we collectively know less a out crypto and security. Second thing it does is that it selects stick-in-ass rule followers away which is exactly contraproductive. And third, it makes us stuck with crappy convoluted code crypto libs had twenty years ago, cause supply of people capable to improve it is not build up.