Its because most normal sized people have to sit at a slight angle to the controls just so they don't nick the other persons cockpit space.
I usually ended up with half my back up against the door, one hand over the back of the seat holding onto the fire bottle just so the student had some elbow room to make a pigs ear of it.
After a few lesson's you both know each other so without knowing it you cuddle up and don't mind infringing each others space. Although being a big shouldered lad with some students we wouldn't have been able to get the door shut if we had both sat straight on.
The only comfy way to fly one is by sitting in the middle and using a pedal each side for the rudder. Has the advantage as well that the CofG is in the middle so it fly's in a straight line as well.
Where as in the PA38 you only infringe the students space when they have a fat arse and you need to get at the trimmer. The rest of the time you have more than enough room to sit/slouch in what ever position you like. The other thing as well is that the PA38 seat travel is huge so you don't end up having to ram your feet under the pedals or get sore legs being unable to stretch them.