I was wondering why there are no players that play forehand and backhand one handed but not both with their main hand.
I don't follow that sentence. Don't players that use a one-handed backhand in fact use their main hand?
Would one not have the better reach if one would play one handedly on both sides with the outside hand?
So make every shot a forehand by using both hands? Yes you would potentially have better reach, but you'd have to take time to do it. The game today seems to favor the power and control of the two handed over the extra reach of the one-handed, with some players using a two-handed forehand.
While the grip changes in normal play between forehand and backhand, most of the movement is just a change of the racket rotation in the hand. The dominant hand never comes off the racket and remains at the bottom of the grip.
To hit a one-handed forehand with your non-dominant hand, you'd have to take the dominant hand off and slide the grip down. You'd have to repeat that when the ball comes back to the other side. I'm sure you could get better at it, but it seems like a lot of extra motion that has to be done before you can get the racket into a good return position. So you're playing reach against speed.
I was pointed at Cheong-Eui Kim, who was peaked just inside the ATP top 300 in 2015. There is some video of him playing with forehands on both sides.