re: Good question
Why / why not indeed ??
I have seen some old films also and when I saw them it seemed to me that the "engineering" of the writers was the idea that by accelerating the craft to a very high velocity before igniting the rocket engine there would be a saving in the amount fuel that would be needed to get the craft into space.
The idea seems reasonable to me, if you can reduce the amount of on board fuel needed to get the craft into orbit, then the useful pay load could be increased by the "saved" amount of fuel weight.
I have no idea why no one has developed the idea further.