Because it involves deferred gratification. Hair plugs are not cheap even for someone with Pig's income and can be painful + uncomfortable right after the treatment. Why do that when he could fork over the same amount of money to WWE Gacha and get the "You Win!" screen right away?
We are also talking about someone who is addicted to his creature comforts-in his mind all the shit he wastes money on (frequent restaurant meals, multiple streaming services, probably a half-dozen other things I hadn't thought of) is a "vital necessity". So in his uniquely retarded worldview, there's no spare or extra money he could put towards hair plugs.