In the spirit of scientific enquiry I've looked at the four MIL threads linked as 'similar threads' below. As follows:
“PIL started inviting themselves to stay in our home for between 3-5 days every 3-4 weeks minimum & often would bring extra people we didn't know were coming. They would spend these visits ordering us around, taking over my baby as if they were the parents, undermining me, ignoring what I told them about baby and doing what they wanted anyway, treating us like kids (we are in our 30s and very independent). I was very overwhelmed with all the additional cooking, food shopping, cleaning and laundry that these visits caused.”
... plus a lot more
www.mumsnet.com/talk/am_i_being_unreasonable/4607025-another-mil-thread
“My MIL ( who I get on very well with and who is a fab grandma ) has seriously pissed me off but as it is almost a one off I feel I should let it slide. But then I'm a resentful old cow so maybe it would be better if I said something.
Incident was that MIL was at our house this morning ( helping as she was asked due to a car issue - she is great like that) and then she asked my 10yo DS "Do you have your bag" - as they were leaving for school - "and have you put your lunch and everything in it?"
My DS answers "yep , it's there at the front door and mum packed it for me"
And then she YELLS ( and she is an ex- school principal, so she can do the loud voice really well)
"You are such a spoilt brat!" Twice.”
www.mumsnet.com/talk/am_i_being_unreasonable/4594289-i-never-thought-i-would-start-a-mil-thread-but-here-we-are
“MIL only looks after ds once a week and, although very grateful, it’s making me uneasy. Her and FIL don’t listen to guidelines and, when we’ve picked him up, he’s been outside (on the front garden) in his pushchair, alone. He’s not even been out for a walk and left outside to continue sleeping! Just put in the pushchair and expected to sleep.”
www.mumsnet.com/talk/_chat/4636118-mil
“When ever I see my MiL she always makes a comment about my weight or hair or both on some occasions. In the 20 years I have known her not once has she been kind to me she bullies me and says unkind things about me . I have always been kind to her and never given her any reason to be unkind to me . My husband has only just started saying things like like her weight is because of her meds but this is only because I said to him it upsets me . For 18 years he never said anything he just let her carry on even though he knows it upsets me”
www.mumsnet.com/talk/relationships/4564438-mil
As a random-ish sample, make of them what you will, but I'm not sure these bear out the OP's theory that it's usually the DIL being precious and unreasonable.