I'm not sure, exactly. Maybe because it's the first thing that pops into their minds? Or they're just momma's boys or girls. :] I've heard "dad" or "mom and dad". Generally it's the mother who is nurturing and cares for her children while the father works hard. In some cases some fathers can be lucky enough to spent quality time with their children and are just as loved by those children. These are not "momma's boys or girls" but kids who respect their parents and perhaps may be excited as to where they are or simply are homesick.