Rude to MIL

Why is it that so many women complain and bitch about their MIL but lord forbid someone say something about their own mother. I love my MIL not a fan of my step MIL but I'm still nice to her bc at the end of the day she means something to my SO. I don't get it. Women on here complain about stuff that their MIL do but I'm sure their mother is doing the same thing and they are just letting it slide. To me, it's disrespectful to your partner. Why should your parent be more important than theirs?!?!