Tradition? The manufacture didn’t offer a gasket, so dealer techs needed to learn to use RTV.
When RTV is used properly there is no seepage. Each time a customer’s vehicle came in with a leaking aftermarket pan gasket, it supports the belief that RTV is the better choice. An unscientific observation, perhaps if I installed 100 aftermarket pan gaskets, then inspected for leaks a year later I would have a different opinion.