The refusal is likely tied to liability, in that they see any aftermarket parts or OoW repairs as a potential failure point and fear the customer may blame them for it. It wouldn’t even matter if the quality of work or parts is as good or better. There’s also another factor of trying to oversimplify any repairs/service to allow them to eventually hire unqualified/uneducated ‘techs’ for things who don’t have a clue what’s going on.