Power.
It corrupts, and it attracts the corrupted or corruptible.
Some people become refs because they are petty little bitches who like showing everyone who is boss. Others because they like to be the center of attention.
Those are just a couple of obvious reasons.
There have also been multiple scandals where refs were being paid. That is less likely in women's basketball because no one really cares enough to bet on it. But it has happened at every level of men's basketball. Other times there is a tacit agreement to protect the rankings of certain teams because huge amounts of money are involved such as in SEC football. Or it could be political with refs effectively cancelling coaches and programs who they feel do not subscribe to the correct orthodoxy on one issue or another.
But just liking or disliking a particular program or coach is more than enough motivation. We live in a world where people become nurses or doctors because they want to be serial killers, pedophiles seek out professions which interact with children etc... Refereeing with bias because you like the feeling of power is small potatoes compared to that and yet all of those things happen constantly. It would be naive to think that there aren't people who enjoy determining who wins or loses, or seeing what they can get away with.
No, the far more difficult question is how could a human being officiate without bias? Such a thing is not possible.
Also, there are psychological effects such as the tendency to see only what you look for and miss other obvious things as a consequence. Most of you have probably seen the experiment where people are told to count how many times the players in white jerseys pass the ball to each other, and everyone who watches the video with that in mind misses the guy in the gorilla suit who waltzes through the group of players and waves at everyone. This is because the viewers are intent on looking for what they have been told to look for.
Of course, it would also help if we didn't recruit most of our referees at the Alcoholics Anonymous Chapter which meets on the campus of Gallaudet University.