> The employer doesn't know how much you'll work, if you'll quit early or give up or make mistakes or not bother to show up.

Knowing salary is also useful for existing employees, and in that case (in the US) the employer likely does know all of those things.

This is true to an extent...but most people with a job are not looking to get fired while most employers are looking to cut costs, and from my experience most employers are just fine to spend 3 months looking for a replacement, only to pay them more(this is bewildering to me). In my opinion this kind of 'war of attrition' hurts both sides and is a sub-optimal option.