Great explanation. Probably where I am from there are more statutory or other costs involved in non-voluntary redundancy so it makes more sense to incentivise anyone to leave who wants to do so.
If a company over-hired in non-core areas during the last two years because valuations were high, they might want to shed non-core workers that they feel that they don't really "need". But they definitely don't want to lose the best people working on the core business.
The people who take voluntary leave are often the most able to quickly find a new job (i.e. the best people). So it is not in their best interest to incentivize that.
A) Voluntarily leave and get some sort of severance.
B) Wait and gamble that the company decides to ask you to leave.
If you wait and are not selected to leave, you likely won't get any severance. So if you are angling to leave, it's safer to just take the package.