Well, he probably learned it from his dad, who in turn learned it from his dad, and so on.
This thing could go back hundreds, possibly thousands, of years.
At that point, you aren't just talking about a fair dollop of casual racism, you are talking tradition, you are talking culture.
You are talking about a decision.
Do I follow in the footsteps of my ancestors, who, through all their happiness and sorrows, their trials and tribulations, the grand summation of all the energy and collected wisdom of their lives, in succession, finally produced me?
Or do I listen to some lisping, heteroflexible anon on the chans telling me that they were retards?
I think I'll go with my ancestors.