It's seems to be mostly a US thing (it doesn't seem common in Western Europe either). It boggles the mind how spending several monthly salaries on a ring became a solidly embedded tradition over there.
You seem to believe the commercials more than most Americans do. The vast majority of people don't spend that much on an engagement ring (I'm pretty sure the commercials said "two months'" not "several" anyway--but still, that's nuts, no one I know spent anywhere near that much). An engagement ring is pretty entrenched as a tradition, sure, but not the boggling part about the expense.