Slowing Down The Past

After having just watched a lot of jerky, speeded-up World War I footage, the thought occurred to me: Why not digitize the frames and create interpolated frames between the real frames?  Then slow everything down and presto -- natural motion.

I assumed that early film technology moved too slowly -- that early film cameras couldn't blink fast enough to keep up with reality. Boy, was I wrong.  

Old movies look jerky because there is a fundamental mismatch between older projection speeds and modern projection speeds and television. People in theaters back then actually saw movies that looked natural and not jerky.

Why is this so hard to fix?