Because he is the superstar. If he is going to want to take the credit for when they win, then he will be responsible for the L's pilled up. Same theory goes for every superstar, and the QB's in the NFL. If you get paid the most, and are the best player on the team, you will be responsible for the W-L records. Fair or not, its the way it is.
You keep ignoring the fact that basketball is a team game. Just because the Kings haven't had a winning season with Cousins, it doesn't mean he is not a winning player. It's seriously short sighted to look at it that way. For instance, if Cousins was playing with 4 other toddlers on the court, he would lose every game. Is that blame really on him? Does that make him any less of a star? Of course not. Little Johnny & Timmy just can't stay in front of their man on defense. This example is obviously an exaggeration, but you can still apply this argument to his past teammates and coaches.
As Brick pointed out, the numbers actually show that we do win when he's on the floor. If you take away Cousins, we are as good as the Sixers. Cousins presence alone increases our win total by around 20 (record w/ Cousins vs. record w/0 Cousins). I'd like to see you find 5 other players who have that impact.
Look at this way..
Player A takes a team that would win 40 games and gets them to 55 wins.
Player B takes a team that would win 10 games and gets them to 30 wins.
With your logic, you would say that Player A is a winner because his team is over .500 while Player B is a loser who you'll never be able to win with. Obviously, this line of thinking is completely incorrect. Player B produced 5 more wins than would Player A, yet you still would like to call Player A a winner because he has better teammates/coaches. SMH