You're right, the engine mfgs don't have anything to gain, but the automakers do.
Dealerships and service centers have huge incentive to give regular oil changes.
Each oil change is revenue and the opportunity to sell more goods and services.
Why do you think the 3 months / 3000 miles scam is still present?
Depends on what you think scam is defined as.
First, engine manufacturers are the auto manufacturers for the most part......very rare to have an auto manufacturer not make their own engines. Sure, you can pull out the rare examples, but as the whole, auto manufacturers make their own engines.
Second, the 3K oil change routine has gone by the wayside....outside what most manufacturers term "Severe duty cycle" or "Short trip/city cycle".
Those are defined as towing heavy loads, the
majority of the vehicle's use is for trips less than 5 miles and/or extended idling in traffic with stop-and-go driving, delivery/police/taxi/etc. service, etc.
With that type of duty cycle, routine 3K oil changes would NOT be a scam. Of course, most people don't fall into that type of driving and fall under the extended service cycles, which typically sees 7.5K oil change mileage, etc.
Just depends on what you do with your vehicle. Of course the Jiffy Lubes of the world neglect to mention few people don't really need to change the oil every 3K miles, but that'd eat into their profit margins. At that point, falling for their marketing is just a consequence of the vehicle owner's stupidity for not reading the vehicle's owner's manual and following the recommended maintenance schedule applicable to their driving habits.