It takes special dedication and focus to learn the rigor on your own. However, once you are able to naturally recognize what's rigorous and what's not, I think

It takes special dedication and focus to learn the rigor on your own. However, once you are able to naturally recognize what's rigorous and what's not, I think it's possible to do it on your own. I think the issue is not how hard it is, it's just that most people have never done anything like learning advanced mathematics before. If you're on your own, you can't learn math by autopilot and reading and regurgitating and allowing the info to naturally propagate through your brain. You have to learn to focus and interactively refine your own version of mathematics.