I always wonder why no one plays with forehands on both sides, i.e., a right-handed forehand and a left-handed forehand. I think that would be such an advantage.
It reduces reach, which is a disadvantage, and most pro players have sufficient swing power on the forehand that the second hand would be superfluous.
Of course Monica Seles utilized the two-handed forehand to great effectiveness.
On the men's side, Fabrice Santoro was probably the most famous and successful two-hander.
It's just different angles and motions and muscles from the forehand and backhand. Physiologically, it's easier to generate pace from the forehand, especially using the rest of your body. On the backhand, the second hand can give you a little extra to make up for the "awkwardness" of the stroke.
I think the real answer is the switching cost between hands. Not enough reaction time to swap hands in the middle of a play.
The answer is that most people are dominant on one side, right-handed or left-handed, and their non-dominant side is much weaker, so a right-hander hitting a left hand "forehand" would be much worse. That's just how our brains work. :-)
The thing about baseball is, all hitting strokes are two-handed, so it's more analogous to the two-hander from both sides than the one-hander. Also, baseball hitters don't have to run to the ball. :-)
I would guess that the second hand in baseball helps a lot with stability and accuracy, because a baseball is so much harder than a tennis ball, and the barrel of the bat is so much smaller than the string surface of a tennis racquet.