Well, IMO it's not even that they are usually "dead wood" but that so many workplaces now have multiple tiers for their workers, where newer employees pretty much always get the shaft compared to the old-timers. They are often on a considerably lower pay scale for the same amount of qualifications, they get 401Ks instead of DB pensions, they don't get retiree health insurance, et cetera.
And this is when the younger workers realize that no matter how good they are, no matter how long they stay with their current employer, they will *never* come close to getting the financial security deal the older folks got. And that can lead to resentment in the workplace.