Ageism is definitely part of it, but most people just don't seem to care to learn in general, and of course the incentives are against it.

They'd rather treat the general version of Greenspun's 10th rule as a commandment, and create a new, ad hoc, informally-specified, bug-ridden, slow implementation of some fraction of whatever already addresses the requirement, than learn about how to use some existing tool that they don't already know.

One of my favorite examples is a company that home-rolled their own version of (a subset of) Kubernetes, ending up with a fabulously fragile monstrosity that none of the devs want to touch any more, and those who do quickly regret it.