There is a difference between an oil meeting the manufacturer's required specification, and an oil being approved by the manufacturer.
Meeting the manufacturer's specification requires a certain content of detergents, dispersants, friction modifiers, etc., as well as the API starburst and donut symbols.
Getting a manufacturer's approval requires extensive testing by an independent lab to ensure that the oil performs adequately for the manufacturer's application (engine). And it costs a lot of money just for a picture on the bottle, but usually it results in increased sales. It's just not worth the cost for an oil manufacturer to pay for testing of each and every one of it's various weights and specialty applications. Imagine how many combinations are possible from: 0w20, 5w20, 5w30, 0w40, 10w60, etc. multiplied by: fully synthetic, synthetic blend, 100% synthetic, high mileage synthetic, etc etc. Each variety must be tested separately in order to get approval.