Because we've "normalized" their behavior in our minds. It becomes part of our daily life, so we desensitize ourselves to it.
The other night, I was talking to my son, and as some of you know, I'm on a short-term separation from AH. So DS says, "No, Dad's been drinking, but he's not bad. He hasn't passed out in the car."
????? When it's normal for your spouse to pass out in the car, and he's doing great even if he's drinking every day because he's not passed out... that's normalizing behavior in a sick way.
So, while what other people see as simply not normal, it's become normal.
That's one of the many reasons I'm here--so I can stand back, and, as suki said, see the forest for the trees. (and LaTeeDa is right--I know that my being here won't "cure" my obsessions, but I do think I have been able to get some needed objectivity in the process.