My understanding is Boeing got the jet requirement lowered from the prop 50 foot because the early 707s couldn't make 50 feet on the runways available.
Remember it's your takeoff distance. Most people think of takeoff as breaking ground. Requiring runway and clearway to allow you to get to 100 feet would make the aircraft performance numbers look bad. The manufactures would probably like the end of the takeoff to be 5 feet. "See, you can weight xxx,xxx and still get off a 6000 foot runway."