Because right now, if you dont agree with the social contract thats forced upon you, you can leave. You can go somewhere else, live differently, even just check out and be a hermit if you really want.

If your on a generation ship you dont really have much choice. You do the job that your given, fill the role assigned to you and only have the entertainment provided to distract you. It would also potentially be a situation where your job is inherited. Eg, your an avionics engineer because your parent was before and they taught you what they know so you could replace them. Over a few generations that could easily lead to a caste society where the command elites at the top never have to be as 'lowly' as a simple engineer or cleaner.

"I come from a long line of janitors, my dad and his dad and his dad were janitors on this ship, and my son and his son and his son will be too"