The main reason is maintainability. There is no more cobol developers coming. Existing ones close to retirement or already retired.
The shortage of COBOL engineers is real but the harder problem is enterprise scale system understanding. Most modernization efforts stall not because COBOL is inherently a difficult language, but because of the sheer scale and volume of these enterprise codebases. It's tens of thousands of files, if not millions, spanning 40+ years with a handful of engineers left or no one at all.
We're exploring some of this work at Hypercubic (https://www.hypercubic.ai/, YC-backed) if you're curious to learn more.
With the current reasoning models, we now have the capability to build large scale agentic AI for mainframe system understanding. This is going beyond line-by-line code understanding to reason across end-to-end system behavior and capturing institutional knowledge that’s otherwise lost as SMEs retire.