Can you offer any empirical evidence that it is not difficult? The vast majority of people I've encountered who are entirely self-taught beyond typical high school (or college freshman) material have been quite bad. They repeatedly make fundamental errors of the sort I would expect to be swiftly corrected in undergrad coursework, even in topics they (claim to) have studied for years.

I don't think I've met anyone who learned to write a good proof without significant human instruction. That is probably something to be expected because much of what makes a "good" proof is social expectations, but I see a lot more cases of unjustifiable leaps of logic than excessive, obvious detail.