Explanation of steric hindrance and existence of PI5
The simplest explanation is that you can not fit five iodine atoms around a phosphorous atom.

Consider the pentachloride equivalent - this is already quite crowded. Since chlorine atoms have a smaller covalent radius than iodine, you can get five of them around the phosphorous. Of course, all this depends on the bond length of P-I and P-Cl and the size of the phosphorous atom.

Interestingly, there seems to be some suggestion of an Uranium Pentaiodide compound, for example this PubChem compound. However, there are no references on that page, so presumably this has never been synthesised either?

In general, steric hindrance means geometric conformations of a molecule that are impossible (or difficult to achieve) due to clashes between groups and atoms within the molecule.