Rudimentary Firearms & Late Game Age of Discovery

So I know guns in AtE have been an often discussed topic, but most of the time I think people are discussing modern machined firearms. The common excuse is that, without an industrial base, firearms that we have today could not be made. In that I agree. What doesn't make sense though is why we couldn't have pre-industrial firearms considering that rudimentary firearms have existed in Europe and China well within the confines of the medieval period.

Guns are like Pandora's box; once you discover how to make them, the design premise is so simple that even a agragrian societies can make anything from rudimentary hand cannons to 18th century level guns without industrialization. Metal tube plus gunpower plus object as ammo. It can't really​ be unlearned.

Obviously we must have contrivances like these to make the medieval setting/portion of after the end work, but could a age of discovery late game work? We already can discover bombards, why not arquebuses (superpowered archers)? What is your headcannon as to why pre-industrial guns don't exist?