Some other good points have been raised about what makes it complex, I will add another. I never learned Cobol, but I learned programming in the 90's on the AS

Some other good points have been raised about what makes it complex, I will add another. I never learned Cobol, but I learned programming in the 90's on the AS/400 where Cobol and RPG were the primary 2 languages. I obviously used RPG. They are both easy to learn and powerful for what they were meant for but they are also commonly used to create massively monolithic programs where massive transaction programs are all written in a single file or with a lot of "copybooks". It makes it a lot harder to reason in your head what is going on as well as the side effects. Things like unit tests are just non-existent so it is just difficult to make changes because of the potential consequences. Especially when these monoliths are handling such important data. We wrote our RPG in a very API driven fashion where we avoided monoliths. I think you pay a small performance penalty for this but it always made our code easier to maintain. Of course it also takes a very long time to compile these programs so the Edit/Compile/Debug cycle is not fun or productive.