The parsimonious reason is that people write songs about comfort and large, happy families, not about AGIs and statistical households of 3.3 to 3.7 people with

The parsimonious reason is that people write songs about comfort and large, happy families, not about AGIs and statistical households of 3.3 to 3.7 people with 1 to 1.44 breadwinners. Songs (and media) reflect cultural values, not cultural realities; see also rosy movies about the "good old days" of 1960s USA being not particularly representative of the period's racial and economic strife.