they are nice people.
What would you do if they were not nice people? What would you do if they belittled you and demeaned you every time you saw them? Suck it up and go anyway?
What if they wanted to give you no independence and choices, but wanted you to live entirely to their will. What you eat, how you eat, how you raised the children ('the' children rather than 'your' children, because in the toxic scenario, you're just a walking womb and the children are 'theirs'). Everything you do gets comment and judgement and defiance.
These might be hilarious 'jokes' but they're the same jokes that get made all the time because 'silly mummy is so sensitive'. They might feed your vegetarian children meat because 'silly mummy doesn't want you to go strong.' They might cut your son's hair because 'silly mummy's made you look like a girl.'
Would you still keep going if this happened every time you saw them?
You also have to realise that your children and grandchildren might not have your view.
I do realise this. I completely accept this, on account of knowing that my children are neither an extension of me nor a possession. At the moment, they are not old enough to make choices, so I make them for them, and I'm going for a) demonstrating that their mother has value and worth and doesn't have to put up with being crushed and b) protecting them from the sort of manipulation that comes with my father.
Those are undoubtedly the right choices to make, largely because I don't want to a) demonstrate that their mother is valueless and irrelevant and must put up with bad behaviour from anyone and b) run the risk of them being hurt or damaged.
I also understand and accept that there may come a time when my children don't want to have a relationship with me. It will hurt - obliviously it will be like a full body blow. However, I'm working on avoiding that situation by treating them like human beings. Ultimately, I want them to be independent. That's basically the way in which I differ from my father. My independence terrified him.
I struggle to understand how people think they are marrying a partner and not getting his family too.
You seem to be under the impression that lots of people think this way. Generally speaking, people try to have a relationship with their partner's family. If it doesn't work, they withdraw.
Personally I struggle to understand how anyone in this day and age thinks that everybody ought to just put up with crap from people just because.