Because we live in a quantum world and there is no universal principal.
Or because using a universal principal would be not be practical.
What universal principal is going to show how many runs were saved better than something based on RE24? Using an RE24 based weighting is basically equivalent to rolling a die and predicting that each number will appear 1/6th of the time. (I suppose you could argue that that is accessible by considering the system without regard to actually rolling a die 10,000 times and recording the outcomes).
By "universal principle," I meant something that occurs each and every time. A theorem. A constant. Hitting a double is worth two bases forever and always. Assigning fractions based on where it was hit, how hard, how well the fielder fielded, and how well the base runner did on the paths is nonsense. It's also foolish to assign decimal values based on every possible circumstance he could have hit it. When issuing runs to outs made, there's no concrete number. I would buy into it much more if it was a given that a run is undisputedly worth X outs.
Why bother trying to convert outs to runs anyway? Why not measure a fielder based on getting outs? The mathematical correlation does not exist
But as for rating it by bases, well that's where I started my own sabermetric search maybe 12 years ago, but a double is not worth twice as much as a single, a triple not 3x as much etc. I think this can be seen by anyone watching baseball for a period of time. Not only are not all bases equal but certain bases are specifically not equal to others in general.
Your Second Base Coach
If Rfield was a better stat, and actually measured fielding ability, we would not need Rpos.