That's because the normal use of a bat is to hit a baseball or softball traveling 60-105MPH with nobody attacking you. There is zero advantage to using it one-handed for this purpose because it costs you power and control. This is ESPECIALLY true when bunting or if you're trying to not "come around" on a swing. Almost impossible to do without both hands.While that's true, the normal use of a baseball bat is 2-handed - although they clearly can be used 1-handed if need be. This puts them closer to the katana, which is effectively a masterwork bastard sword. So I think Neonagash's analogy is a good one.
Even even if you were using a smaller club in a baseball game, you'd use it the same way. See stickball (as well as spaldeen/fungo). Traditionally, it has been played using a broom handle, but there are manufactured bats for it: 1 yard long and 16oz. And they're still used with 2 hands for the same reasons- power & control.
As a weapon, the bat is perfectly fine in length, balance and mass to use with one handed with shield or with a hand free for grasping/grappling, and requires no special training to use in that way.