Because of the fact that most people that do this "mod" do a REAL horrible job of painting their hubcaps, that it is looked down upon when suggested. It's not something that most people could do properly and wouldn't want to waste money on to have it done professionally.

But when done to wheels, more often than not they're powdercoated, and not painted. This lasts longer than conventional painting and it wields excellent results.

Best suggestion is to leave them alone. Just save up and for a nice wheel/tire combo.