We really should examine this assumption.
Was bullying a constant problem for youngsters when most children were homeschool or merely attended a highly age-integrated one room school for three or four months of the year, lived on farms, and were nearly always in the constant company of a caring adult?
I do agree with you that it commonly happens where there are close concentrations of people, especially when they are segregated by age and with no hope for escaping the environment. ( compulsory K-12 socialist-entitlement schooling).
I was listening to Glenn Beck on the radio talk about the bullying problem, and he made a good point.
It was a long yammer, but the gist of it was that we aren’t setting a very good example for our children - all the bullying that is being demonstrated by our politicians, media personalities, etc.
Children do not learn from what we say, but from what we do. Our social discourse is teaching them that bullying is an acceptable method of intimidation in the quest for power.