Why aren't computables used for numerical calculations?
It sounds extremely inefficient compared to floating point. We have a very good understanding of how to control the errors in floating-point calculations (e.g., adding small numbers before large ones, avoiding taking the difference of large numbers and so on) so the only benefit of the methods you're suggesting would appear to be that they offer increased precision. But, for almost all applications, 64-bit floating point is more than enough and the speed of being able to do calculations with single CPU (or GPU!) instructions is a huge win.