Rewriting existing systems is one of the riskiest things a company can do, so old codebases are going to be around for a long time.
Those old COBOL codebases likely contain 50 years of bug fixes for every possible edge case. It'd take a long time to rewrite everything and ensure feature parity, and there's usually not a significant business reason to rewrite it (after all, a successful end result is just that the system behaves exactly the same as the old one).
The COBOL language is still getting updates, too.