I've been on the other side of two of these and the explicit alternative in each case was bankruptcy. Also asset transfers are more expensive to the acquirer because you have to explicitly delineate the assets you're buying and what you're not buying. This makes for more lawyer time and pushes the transaction costs up significantly. The real reason to do it is because the team there at the time is more valuable as a group than they would each be on the open market individually. Anyone _not_ there doesn't add that kind of value to the deal.

You may want to argue that IP is also part of these deals and past employees created part of that. This is probably true for some deals, but it has been minimally true for the deals I've seen directly. Sample size of two isn't great, but keep in mind the value of startups is largely believed to be in execution, not ideas. A startup on the verge of bankruptcy probably doesn't have immensely valuable IP because it's 1) not producing present value obviously and 2) isn't obviously worth a lot in the future, otherwise someone would be willing to give you discounted cash today for an ownership percentage of its future value (aka an investment).