I actually know the answer to that. It's egoism married to the "its not my job" way of mind.

Some people get into programming because of the money. A mid level developer in America can make over 200,000 dollars. Those developers don't actually care nor love code, they want money. So they learn the skill, which is doing menial tasks in one framework, and that's it. Everything else is "not part of their job".

At one of the startups I worked at (as a project manager contractor) there was a "developer" who was proud if himself for knowing react, and scoffed at learning anything else.

This was a small company, 5 developers. There was a need to write some backend code, the frontend was doing OK, but the backend was in a tough spot, months behind schedule. He was asked to try and help and seemed offended that he had to do anything other than react. He refused, it's react or nothing else.