Because most of them can't. Only so many of senior engineers can become managers (because of obvious mathematics, management is a much smaller fraction of overall jobs available). Most of them get to a point their salaries flatten out and there isn't any advancement they can do. At that point that usually have paid off mortgage and kids so they are content just having better work life balance, doing 9-5 work & collecting their paycheck, not caring too much about advancing from that sweet spot.