My hubby’s a sr staff software engineer- he’s 52, leads teams for a start up. He’s seen it all. Was a Flash designer when that was big. I think the older engineers get sick of all the managers who don’t know how to code. They get burnt out. My husband does. He’ll work a job for a couple years then work on his own apps thrm get back into a job. The best engineers often get burnt out because they are often not given promotions because they’re needed in the trenches. Yet leadership never really asks their advice and owners and managers are threatened by their brilliance . This pisses them off. Then they rage quit. My experience.