Having worked as software engineer for big it companies, the job offer looks like a contract - are you referring to something else than a written job offer contract?
Yes. Ask a senior executive in your company if you can see their employment contract. Doctors and lawyers usually have written contracts tailored to their specific requirements and responsibilities. Professional athletes would never work without a contract.
A job offer is an agreement for $x compensation for x hours worked.
A contract specifies what that work is and is not. If it's not in the contract and I don't do it, well that sucks, for my employer. Without a contract I could be fired for cause.