Because the company is usually offering some sort of severance, which will be withheld if you say "Yeah, no".

They're not saying, "you're laid off, effective today, but you have to stay for another two months", they're saying "you're laid off in two months, and your job duties are now to train your replacement". Saying "no" means resigning.

Because you get a runway to find a new job, you get a paycheck while you're on that runway, AND you get a severance at the end of the rainbow to tide you over if you haven't found that new job (or a killer bump to the savings/nice vacation/whatever) if you HAVE found a new job by the end. In addition to your PTO payout, or whatever.

Else if you flip your desk and walk out, you get your PTO and now you have no job.

Cause then you don't get your severance. And few people are prepared to lose their job out of the blue like that.