Seed oil got grandfathered into GRAS because they were in US foods before the regulatory regime started.

The marketing is largely Ansel Keyes & the vegetarian diet doctors (many of whom have religious or ideological convictions promoting vegetarianism). They vilified saturated fat & had to promote something else. Another key proponent was USDA, who was tasked with promoting produce and then with setting diet guidelines. Shockingly, the one task influenced the other.