Well, because basically we have to take into account the acidity and basicity of the solvents in regard to that of the solutes (i.e. their likelihood to give up or receive a proton compared to the other or how strong is their ability to receive a proton?) And to broaden up, it's not necessary to be between a solvent and a solute but two substances with same regards as above. An example is NH3 and HCl in benzene, HCl gives up a proton and becomes the acid in this context. There's no dissociation of HCl to its ionic components, just a simple proton transfer.