There’s no mystery. You’re thrown into a ‘family” situation with people who aren’t your family, and whom you often wouldn’t give the time of day to unless for the accident of who you happened to marry. You may have equally maddening family members in your own family, but often you’ve learned to accommodate those over time and from long affection/duty, none of which pertains to ILs. Your spouse/partner will have a different set of relationships with his parents, and a different set of accommodations, which you may not like.
Add in gendered expectations about DIL’s ‘managing’ family relationships, and ‘controlling access’ to grandchildren, and you have a fruitfully complicated situation.
Eg. I ended up having to speak extremely plainly to my MIL who kept getting cross about me not calling her back about some family party planning stuff, and tell her I wasn’t DH’s social secretary, I had a demanding professional job, like him, and that, like him, I didn’t give a thought to whether I was going to Uncle Paul’s barbecue or whether DS needed new shoes between 9 and 6, Monday to Friday because I was on work mode.
I don’t think there was any malice, particularly. It’s just that MIL never worked, neither of her daughters worked after marriage, and she seems to have thought that marriage and a child in my case meant I must be magically available for shopping and bingo on a weekday, because in her head mothers don’t work. So she sort of ‘unsaw’ my career.