Most American workers have an employment agreement with generalized clauses to the effect of "we can terminate you at any time for any reason" and "hours are thusly unless management needs you otherwise" and "duties are so-and-so but also anything else we want you to do."

I've had jobs where I kept the software running so smoothly 80% of my time at work was spent with a broom or a rag cleaning because "downtime" was not a thing.

Federal employment laws, state employment laws, and court precedents substitute for a detailed contract for many employees in the U.S.

This is in fact a big reason that companies like to use so many contractors: it’s way easier to manage the downside risk for a contractor because the employer’s liability is scoped entirely to one written agreement. Whereas their relationship with employees is scoped to the broad set of laws and precedents mentioned in my first paragraph.