If SOCl2 reacts with alcohols via SNi, why doesn't POCl3?

The reaction with SOCl2$\mathrm{S}\mathrm{O}\mathrm{C}\mathrm{l}{\phantom{X}}_2$ is also used instead of PCl3$\mathrm{P}\mathrm{C}\mathrm{l}{\phantom{X}}_3$ and PCl5$\mathrm{P}\mathrm{C}\mathrm{l}{\phantom{X}}_5$ when retention of stereochemistry is required. Phosphorus trichloride and phosphorus pentachloride both lead to inversion.

The mechanism proceeds via a SNi$\mathrm{S}{\mathrm{N}}_{\mathrm{i}}$ pathway, or internal nucleophilic substitution. This step has been highlighed in red; an intimate ion pair[see below]${}^{[\text{see below}]}$ is given in square brackets. Adding a nucleophilic solvent such as pyridine considerably increases inversion with SOCl2$\mathrm{S}\mathrm{O}\mathrm{C}\mathrm{l}{\phantom{X}}_2$ via an attack at the sulfur atom. Green marks blocking of the intramolecular substitution; blue is for inversion.