Because it assumes that their desire for an inviolable mutual meeting time is the only factor in the world that matters. It places their own plans on a pedestal above all other factors that might influence the plans of others.

Not that many will be reading this thread at this point, but for the record I'll add that punctuality is a means to an end, not an end in itself. Those who obsess over punctuality are inconsiderate, at best, of every other means, end, demand, and desire.