Theres reasons. One, we just sorta didn’t know. In less than 100 years we went from single shot flintlock pistols to repeating revolvers; we were still figuring a lot of things, like safety, out. Modern revolver lockwork would take a few more decades.

Even today, in a world of very high quality modern firearms, safety issues arise. The Sig 320 - now the M17 service pistol of the US Army - had a problem where if dropped just so, it could discharge. It was kind of a scandal; modern firearms are incredibly safe to handle, compared to even 30 years ago. Consider that in the 60s it was not unheard of for pistols to require the user to pull the trigger as part of operating the decocker mechanism. Insanity to modern eyes.

Anyway, firearms at that point were kinda new. We didn’t know what “safe” really meant, we didn’t do things like drop tests, we lacked the material science to make carefully heat-treated components, we lacked the manufacturing ability to make the sorts of small, high quality components we use today ... it wasn’t a design defect as much as we didn’t have the science and understanding.