Because a service is one of the most simple things to complete on a car isn't it? (Valve clearance aside).
It's likely an Indy will be or will employ experienced staff (possibly with a trainee as well) that will complete whatever work they get through the door.
An Indy will live and die by its reputation. A dealer will get work regardless of reputation from fleet cars, warranty work etc.
My money is with a (well respected) Indy. I'm confident I and my car gets a better service!
