Why are plants unable to take up Phosphorus directly in their organic form like Phytic Acid?
I am not a plant biochemist but would make a couple of points that may or may not be relevant or helpful.


The living organisms that preceded plants had evolved a wide repertoire of systems for transporting ions across membranes, involving variations on a few general designs. Given the importance of inorganic phosphate in living organisms, phosphate transporters would have evolved early on.
Phosphate may have been more abundant in soil before sophisticated evolution of plants led to secondary metabolites like phytic acid. If this were the case it would explain why roots developed phosphate transporters.


This is not to say that it is impossible for organisms to develop transporters for more sophisticated molecules — there are many such examples — but in this case such a development would only be expected if there was such a lack and intense competition for phosphate that it would convey an evolutionary advantage.

This sort of question is always a matter of conjecture. One imagines there is a tipping point when the difficulty of developing a new system is outweighed by the advantage it would bring. This clearly hasn’t happened, but which of these factors is most important is difficult to say (for me, at any rate).