It's not Cobol programmers that are rare, it's a shortage of people that can understand the business logic. Cobol is often used in very complicated business operations.

As others have pointed out, probably more the intricacies of the existing business logic, plus the understanding of the mainframe systems that it usually runs on. Otherwise you could just build a cross compiler and call it day.

Those who do are valued not for their programming skills but their knowledge of the (very complex, largely undocumented) system and its business environment.