First of all the really wide, open, coamingless cockpits are racing cockpits. These cockpits are designed to allow the crew to work efficiently, getting them outboard where they can see what they are adjusting. Partially because of their large surface areas they went to wide open transoms allow quick drainage of any water that happens aboard. This design is not just used on inshore racing boats but also on crewed round the world racers who often venture where few folk dare to go, approaching the antartic circle.
You see something different in coastal cruising boats where their cockpits have been designed to be comfortable for small armies to lounge and they will have removable transom panels of one form or another to make it easy to get into dinghies or to swim. For some reason the ads for these boats often show them with the tansom doors off or stowed. These coastal typically do have substantial coamings.
The cruising versions of "Moodys, Freedom, J boats" do have coamings and closed transoms. Even the racing versions of J-boats have closed transoms, which is more about proper rudder post support than seawater control.