This is the actual incentive, from a researcher's point of view. Even when there are plenty of outlets that provide quality control, archival, etc., researchers still submit to the same traditional journals because those journals' brand names are the stamp they need on their CV to advance an academic career that is already incredibly unlikely (due to their being far more applicants than positions).

It's not the only way, but an easier one if the scientist just cares about their own ass.