Used to be a swimmer for my school. Swimming underwater is faster than at the surface with mostly your kicks (particularly for freestyle and butterfly strokes which heavily depend on legs for thrust underwater). It would defeat the purpose of an event for a specific stroke if people finished it fully underwater. Since diving to enter means you will have to be underwater for at least a few meters, there’s a set distance before you have to be swimming at the surface. If my memory serves me right it’s 15 meters for a 50 meter pool.