Because most founders who made it in the field, did so at a young age and so they are biased to view old people who didn't "make it" and are still coding as incompetent even though it has nothing to do with reality. Reality is that being good in the tech side does not correlate that mich with being good on the business side. They are almost independent factors aside from some low baseline requirement of competence...

The baseline requirement of technical competence for extreme financial success in tech is so low that most big tech companies don't even hire rank-and-file engineers whom don't meet that requirement half-way.