I think that this is people finally getting fed up with corporate hypocrisy. When you quit, for example, it's expected that you give at least two week's notice, and anything less would be "unprofessional". When there's a layoff, though, people are shown the door immediately (or sometimes even fired over the phone). Why should I be held to rules that don't apply to the company that I'm working for?
What's the primary thing the employer gives the employee? Money.
I think it's reasonable when one party decides to end the relationship, that they keep providing that primary thing for a period of time to make the transition more orderly.
That makes notice periods and severance periods logically equivalent in my book. The company can show you the door immediately (if they want), but as long as they pay you severance for a period of time, they've held up their end. (I've never seen zero severance in the tech industry unless a company is literally out of money or in the case the employee is being fired "for cause" (typically meaning sexual harassment, or theft/embezzlement, etc, not mere incompetence or laziness.)