Big tech employees are a fraction of all tech employers and employees. There’s problem so systematically pervasive that you see this across the board. Companies do not value long term retainment of employees up and down the size spectrum and it’s very clear in how they behave.

They don’t award experience and depth of knowledge in any way that is indicative of fostering retention

Big tech employees have been doing that because they see new hires (who have themselves switched from another big tech company!) get a lot more comp than their meager raise. Don't blame the workers from reacting to the signals that company is sending through its own deliberate actions.