I think it's because of the disconnect we have in our lives now.
His money/my money, his family/my family, his children/my children etc. That and our increasing lack of communication skills. We are becoming more insular and less willing to compromise. Don't answer the door, don't answer the phone, text first to see if they want to talk to you, and so on. If you have a MIL/SIL/FIL problem, it's not your problem to sort out, get your husband/partner to do it.
There is never just one side to a story, but on MN we only hear the one side. Sometimes if it's obvious the poster is the problem they will post further adding increasingly bad behaviours to back up their stories. In Laws mustn't love your child too much, mustn't buy them presents, mustn't want to spend time with them, mustn't ignore them, mustn't want their own lives, the list goes on and on.
Then egged on by keyboard warriors to go low or no contact, 'call them out' or 'I wouldn't allow that' when really all that is needed is a sit down and frank discussion with both parties.
I'm not a MIL but I was a DIL and I wouldn't dream of behaving as badly as some DILs purport to on here.