First of all, without certification and test data it is hard to just trust a product over another, unless the consumers know what they are doing (a domain expert). So, as long as the customers are buying the certified product, they can't tell which one is better than another (they think they know based on marketing, but it is hard to tell without test data). Car manufacturers know, so they only spec based on the cheapest barely acceptable oil they already certified, with some safety margin.
People with domain knowledge will then buy "better" or "cheaper" by tweaking their own purchases. They will buy extended drain, high mileage, sludge cleaning (i.e. Valvoline Premium Blue Restore), top shelf synthetic (i.e. for racing), different viscosity, etc after they learn what they want to achieve.
So, to recommend the cheapest oil that pass spec makes a good starting point.