Why do the engines leak so much? The problem starts when the engine stops leaking. If a radial engine stops leaking, then it's out of oil, and that's bad.
When pushrod tube seals are clamped in place using band clamps over rubber hoses, when rocker box through-bolts are sealed with o-rings held against bolts with loose enough tension to only require castelated nuts, when there's no end of parts with the clearance to pass oil, when oil congregates in the lower part of the engine by virtue of gravity and design and gets blown out exhausts on start up...you're going to find oil dripping from the engine.
I've spent a reasonable amount of time piloting radial engine airplanes, and a lot more working on them. Whomever thought up the idea of white shirts for pilots around airplanes was an idiot. Especially in times of radial engines.
In motors like the R2600, typical consumption runs about three gallons an hour. Sometimes more, sometimes less. Some powerplants leak or burn less than others, and the type of operation has a lot to do with it, too.
Suffice it to say that when taking on fuel, nearly always one endevors to fill it up with oil, too. We often operated for three or four hours before fueling, and would end up putting in 40 to 60 gallons of oil while refueling.