[pas]

Why doesn't the company announce that they are going to cut about so and so many people in the next this and that timeframe? A lot of folks would go willingly anyway. (As this happens in other countries.)

Managers can pick before the announcement and then HR can do the actual if not enough people left willingly.

And there's always a chance of wrongful termination lawsuits...

... the real reason this is done this way is because that's the culture of these companies, mostly due to US culture (and labor laws are a synergic manifestation of this).

[_b]

People aren't interchangeable. The company might want to layoff 10%, but they don't want to layoff just any 10%. They will want to layoff people in positions and teams that they want to do away with, and to the extent some people are interchangeable, they would like to layoff those that are the least productive.

If you announce layoffs before telling people who will get cut, the people who tend to leave voluntarily are often those most able to get jobs elsewhere because they are high performers in positions that are hard to staff -- so exactly who the company wants to keep.