As far as I understand, because employment is at-will and firing is trivial, most employees in the U.S. do not even have a job contract! This was very mind-blowing to me when I took my first U.S. job, where there was no contract (!!), only a 500-word "offer letter". I guess the reasoning is that if there ever were to be any conflict between employee and employer, the conflict would be settled by ending the employment relationship. So there is no point in the employer promising anything (e.g. number of vacation days) since the employer can costlessly renege on any such promise.