I think it is the stress that helps in many ways, unlike hydro you can dry a plant out to damn near wither and they bounce back in hours. Also as you mentioned the soil grown plant has to in some ways find its food/water more stress........how is it possible go stress them similarly in hydro would be hard now that I'm thinking about it do you lower your RH late in flower in hydro like soil usually yes I would guess but not the same since a plant in soil will evaporate its water content where in hydro your technically still in water so more stress goes to the soil grown plant, but that's assuming lowering RH is a form of stressing a plant.