It's less about knowing COBOL and more about knowing the design decisions, libraries, common functions etc that were in use at the time, or in many cases, knowing the codebase itself (once those guys are gone...no company will pay the 1+ years of salary needed for someone to become well versed in these large codebases).

I had someone who was a bank COBOL programmer explain to me (programmer professionally for 20 years) about this, because I thought the same thing, but the situation is a bit more complicated than first glance would indicate.

That said in the end it comes down to the fact that they just don't want to pay for results. Whether that be paying a retired COBOL programmer who knows the codebases for banks and whatnot a LOT of money for a short consulting and fixup gig, or paying someone who is newer in their career for a year or more to get up to speed on it, potentially build a test network etc.

Software development is considered a "liability" and not something you should do or maintain to keep your business running well, so they just avoid ever hiring any programmers to do anything for as long as possible and this is the result.