Probably because of the things that I mentioned.Not all SCs allow though. I heard mostly not.
Furthermore at SC, you can't stand there and watch how do they work with your car. So, you won't know what kind of work quality that they are giving you.One reason was my brother bought a civic too a year or two earlier and the service was not good, so I forgo it.....lol
Another reason is the trainee thing......hhahahhaha
Forester still under warranty and bought new not those 2016 model offer in 2017, so it come with 3 years full free service includes oil and filter, so of course send back to SC.....lol (All free mah!)
Many SC don't allow outside oil and stuffs.....
3 years full service for free, of course go back SC better. Anyway, new car maintenance also nothing much to do, can trust SC enough to let them handle it.
Yeah, later put wrong type of oil viscosity or brought in 3rd party parts and end up got problem, SC will get blamed. For their reputation and safety, the best thing that they could do is not to allow any other outside products. Besides, using outside products will only lower their profit margin.