A lot of it is about wifework. MiLs seeing it as the DiL's responsibility to organise stays, send cards, bring the grandkids round, keep the house tidy because she did it all.
This. I, on the other hand, do not do 'wifework'. I have a demanding job, divide childcare and domestic chores with DH though he currently does the lion's share because he's working more from home and I'm having a particularly demanding patch at work and regard contact with DH's family, whom I like, as his issue. Our cleaner retired in the summer, and we haven't replaced her yet, so I'm doing slightly more cleaning to balance DH doing all the cooking and shopping, but I'm not particularly concerned by the tidiness of the house.
My MiL and SiLs, all of whom I genuinely like, but who have led very different lives to me (SAHMs, large numbers of children, a lot of importance put in home-making, cleanliness, family relationships and get-togethers, with men allocated the roles of watching sport, saying 'I dunno' is asked about anything other than the offside rule and acting as providers) do not get me, and think that I am a chilly 'career woman' who is blameworthy in only having one child by choice, but that really isn't my problem.