I suppose it’s easy to ignore any signs of it happening from that point of view. Their kids have friends (the clique) who can represent the facade of everything being just fine, which every parent desperately wants to believe in in advance. If they catch a glimpse of it, they’ll write it off as the bullied kid being the problem.
And that’s best case. In worse cases, the parents will encourage the bullying directly or indirectly and see nothing wrong with it.
It’s the same way that things like racism or sexism are taught from one generation to the next. It’s not that anyone wants their child to be so, they just don’t see it happening themselves.
These kinds of behaviours don’t happen because they want to hurt someone. It happens because they want to protect themselves (even if it’s at the cost of others) so they can fit in with the popular people. The desire to fit in comes from fear (or acknowledgement) of not being able to solve stuff on their own. So instead of teaching their children algebra or whatever is necessary to perform, they teach them to fit in at all cost, because that’s what worked for themselves.
So in short, it’s ignorance paired with herd mentality.