I pick up a lot of road grit on my legs that my front tire churns up. Hairy legs tend to make washing out that grit more difficult.
I don't want my legs marinating in road dust, which I imagine consists of everything including asbestos from brake pads, oil, anti-freeze, transmission fluid, rubber particles from tires, oxidized steel, animal and human fecal matter, urine, vomit, blood, trash, used diapers, roadkill ... pretty much anything you can imagine being discarded intentionally or unintentionally along roadways.
I would prefer to have all of that crap getting washed down the shower drain, and not hiding out on my skin and in my pores underneath (un-aerodynamic) body hair.