They don't pay enough. That's the issue. If COBOL paid as much or better for the average developer than Java, there would be no shortage of COBOL developers.
Also, COBOL work is kinda crap. Debugging 20-year-old Java code sucks, but a 40-year-old COBOL project, with source code backed up to 5-inch floppy disks - forget about it. (I just watched the trailer for 'The Many Saints of Newark'. LOL)