i wish i knew. i can tell you when i was laid off (in software), i was given an nda and non-disparagement contract that my severance was contingent upon so based on that experience, these companies consider severance a “gift”
edit: i think it also (duh) depends on the state - many states are “employment at will”, and i would guess in those states since they can dismiss you without cause (save for discrimination), you aren’t required to pay severance. most companies still do, but i imagine the requirement wouldn’t mesh with the concept of at will employment