Why are MIL’s so difficult?

Why do you think 90% of MIL’s are so difficult? Is it because it’s the next closest person who feels like they are our own mother and have the authority to treat us bad? Or is it because they’re protective of their son and purposely treats us terribly because we’re not identical to them? I honestly hate my MIL, and everything about her. It’s almost 6 in the morning, I’m 30 weeks pregnant and I can’t sleep because the thought of my MIL makes my blood boil. This is even after moving 3,000 miles away from her. 😫