I believe the deep lore on that is Phil adopted the tight grease slick during the same era he chose the goatee - which was also at the height of CM Punk's career and Phil's adoration of him. That was the look CM Punk had at the time, and the look immortalized on the expensive signed plaque that Phil still treasures.
I think the look still exists for a combination of signature Phil reasons: His opposition to any and all change, the fact that he doesn't use product so it actually gets easier to slick it back the longer he goes without washing it, and he has discovered that it is probably the most effective combover to hide the balding. It does nothing to help the receding Vegeta-esque hairline, but that is less of a ding to his virility than what we suspect is going on at the back.
He seems to hate when he showers and it is 'poofy', and while it does look stupid, it's also the only time his hair looks 'normal' and clean. No surprise, it upsets Phil. Probably another reason he seems to typically shower once a week.