Because humans don't want to spend time to learn how to use "dead" languages and humans still play a role in programming. Having LLMs churn out more COBOL instead of Java means more of the code base becomes 'terra incognita' for the involved humans. Learning COBOL isn't that hard - which was one of its stated purposes - but it is still seen as decidedly 'uncool' and 'legacy'. Maybe the increased role of LLMs in coding will change this and make coding in 'legacy' languages 'cool' again just like e.g. the availability of accurate time sources like mobile devices made mechanical watches trendy again or the direct access to music through streaming services made vinyl and now CDs (i.e. 'physical media') regain market share.

Maybe.

Still not something you want to risk your business on.