I don't think that the US has the culture that employees should be grateful to their employer for providing them with a job and a paycheck. The idea of at will employment comes from the ideology of freedom rather than who owes who.
At will employment always always benefits the employers more than employees. Your job is tied to your health insurance as well. So there is a much much bigger incentive for an employee to stay at a company than for a company to keep an employee.
