There a lot of people who say "I love you just the way you are", but think "I can mold them into my perfect partner".

Abusive relationships exist, also its not just grey and white. Oftentimes at the beginning of a relationship everything seems fine and happy but as time goes on the bad stuff shows up, one partner can't handle the others adhd sympoms and this kind of conversation can show up

Besides your parent mentioned relatives as well. You don't choose those

Often these "unchangeable" personal faults are revealed after a commitment like marriage, children or signing a lease together.

Now all of the sudden you are a full time in home aide for your former partner.

As for why the person with the problems doesn't leave they don't want to have to spend a few months/years as a contributing partner to trick someone into caring for them again.

These things don't come out on the first date, they can develop over the years at which point you become stuck with that person (could be children, pets, property and other stuff). Many people are simply trapped - they cannot leave because they won't have money to get some place to live, they got cut out of their friends / family and are controlled by fear ("If you leave me I am going to kill myself and leave a note that you did it, you'll go to jail" or "Without me you are nothing, you are worthless, you won't find anyone and you'll die alone" etc).