Too many jobs come with hidden assumptions, and not enough employees challenge them.
While it might say one thing on the work contract, the day-to-day reality of the workplace situation can prove to be a totally different beast. Pressure gets piled on, duties become more complicated without any increase in pay, and the hours sneakily stretch into overtime territory without any recognition of that fact. It is an unfortunate reality that many employers will happily exploit those who work for them in these not-so-subtle ways, so long as they go along with it.
What is even worse is that those who do challenge unfair expectations often find themselves penalized for it, especially if they are going against what is considered to be the company culture. When everybody follows the unwritten rules except you, it can be an uphill battle for your behavior to be accepted. It may even be the case that staying in that job becomes untenable. It does not always matter that you are doing something that you are within your rights to do; if everyone else decides it is incorrect, you may have no choice but to conform to some unreasonable expectations or find work elsewhere.