Or they rely on continued employment for their health insurance (like most working-age people in the US), and even though it's a very good deal, they don't want

Or they rely on continued employment for their health insurance (like most working-age people in the US), and even though it's a very good deal, they don't want to have to pay $1k-$4k/mo (depending on the size of their family) out of that severance package in order to pay for COBRA. They might also be worried about finding a new job, and would prefer to make their exit when they already have another signed offer in hand, even if it means losing out on the extra cash. People make decisions based on a multitude of factors, and your statement is shallow and ignores much of reality.