Millions of Americans (hundreds of millions, really) are still using incandescent light bulbs, a technology that is almost 150 years old.
Millions of Americans are still driving cars powered by internal combustion engines, a 150 year old technology.
What's the problem with a 60 year old programming language? Should these agencies re-write to run on a 40 year old operating system using a language based on another 40 year old language? Hell! maybe they should re-write the whole stack in Rails! We've seen how successful that has worked out.
That old code, with some new code too, running on modern mainframes forms the backbone of the worlds business. Mainframe technology, though seemingly dated and arcane, is extremely robust and supports exceptionally high volumes using COBOL and in many cases, mainframe assembler.
You don't replace something because it is old, you replace it when the economics of not fixing it demand replacement.