Can't. The word you're looking for here is can't. The unit economics of making the sorts of games that JV makes completely preclude it. It would be way easier to spend the money he doesn't make to get better art talent. This is true. It is also not very meaningful.
And it's not that there is minimal sense of direction just for funsies--it's that you don't have that luxury when we're talking about the rates he's working at. I spent a decent amount of money, in the ballpark of what JV probably spends on third-party work for a game, trying to get freelancers--reasonably priced or, in reality, not so--to work towards a cohesive art style. That I did not ship that game (which is still in a 75%-ish complete state in my graveyard, it's my fuck-you-money project) is largely due to the difficulty of getting art to a standard I felt comfortable with.
JV ships because that standard is lower than I was able to stomach.
His games also happen to be really good, too.