TippyTappyLappyToppy and BErtrand there are plenty of posts and MN threads where people complain about their MILS.
Men have been complaining about their MILs (mothers of women) since time immemorial, and the sexist MIL joke that reigned supreme in 1970s stand up routines is about the mothers of women.
It is simply not true that nobody complains about their own mother.
There are posts on this thread about both sets of grandparents being a nightmare.
The difference is probably that a lot of people are able to speak more directly to their own parent, or have internalised the same types of expectations as their own parent, and dealing with your spouse's parents different "norms" (whether parents of the husband or wife) is an awkward situation for a lot of people.
The relationship with mature adults who you acquire as family when you marry their adult child has always been prone to being fraught, and especially these days where people often marry somebody with quite a different background whom they meet in a context utterly separate from their family. Of course this can sometimes make misunderstandings and clashing expectations and different boundaries more of a problem with the family of your spouse than the family you were born into.
But there are a myriad of MN threads where people have deep seated problems and conflicts with their own mothers and fathers and siblings.
All this "its always the angelic mother and the evil MIL, every time" stuff is quite simply and blatantly untrue.