The same thing can be said for Xander's parents. After 'Hell's Bells', we never see them again and they're never mentioned again, not even when Sunnydale goes to hell in S7.
I think the show just liked to prove how awesome and understanding Joyce was by making Xander and Willow's parents the crappiest parents in history, but at least it was consistent from the outset.
Even in S1 we knew that Xander's parents didn't care much about him and he preferred to spend his time with Willow, or at least have Willow there to act as a buffer. At 15, Xander was ordering takeout for dinner because his parents never bothered to cook anything. In S4, Xander's nightmare in 'Restless' was the fact that he could never escape his parents and it was implied that his father beat him. When Joyce dies, he goes and visits Willow's mother, because he doesn't want to spend any time with his own. In S6, the demon prayed on the fact that Xander was scared that he was going to turn out like his father, a wife and child beater.
Willow was the same, being ignored unless her life directly impacted that of her parents'. Her father didn't want anyone to know that his daughter might not be following his religion, her mother, despite seeing her every day, didn't realize that Willow had cut her hair for months and when she did find out, dismissed it because it wasn't important. She doesn't even know the name of Willow's best friend. When she finds out about that Willow is gay, she doesn't care unless Willow is going to do something about. When she realizes that it's not a political statement, she promptly decides she doesn't care.
Xander and Willow's parents just don't care - Xander's because they just...don't and can't be bothered, Willow's because they're intellectual people and only care if it directly affects them or the things they're lecturing about.
The only person in the entire verse who has been shown to have actual caring, loving parents (as in, both mother and father) was Fred.