Why are electrons shared equally when calculating formal charge, but unequally when calculating oxidation state?
The oxidation state is a form to distribute the charge of atoms in molecules considering to have only ionic bonds. In contrast, the formal charge is the distribution of the electrons considering to have only 100% electron-pair covalent bonds.

Actually, the formal charge is not very realistic and is more or less a bookkeeping tool. Now there exists theoretical and computational methods to calculate more reasonably charges on atoms from electronic structure calculations like Hartree-Fock, post-HF, semi-empirical and density-functional methods. 

One example, taken from Anslyn's Modern Physical Organic Chemistry, is the case of the tetramethylammonium. One would place a positive formal charge on the nitrogen atom, but this is more electronegative than carbon, so there is a contradiction. The calculations mentioned before, indicate that the nitrogen is essentially neutral and the positive charge resides on the methyl groups. So the formal charge is more adequate for indicate the charge on the molecule and not on the atoms.