I've often wondered the same thing. Staying in good physical shape is necessary for my profession so I go to the gym five or six times a week. I alternate between aerobic machines (stationary bike, ski-machine, etc.) and the weight machines. I've found that by increasing my aerobic capabilities my ability to perform multiple ukemi and randori sessions has increased. The weightlifting has helped make my technique stronger also. I know you are supposed to blend with uke and use his/her energy or strength to your advantage, but sometimes uke doesn't give you enough. It helps to add a little energy of your own sometimes, at least enough to accomplish the desired technique.