Why should a classical rest on your left leg?
It doesn't matter with other guitars, right?
I see a first post, Welcome aboard!
Frono captured the gist of it, but the ergonomic thing is 2-fold:
Putting it over left leg
at an angle puts the neck in the most ergonomic position for accurate fretting, that's universal no matter what kind of guitar it is except that some larger body sizes may be a bit more difficult to wrap the shoulders around.**
It also allows the right forearm to rest on the edge of the lower bout in a position that puts the fingers directly over each of their corresponding strings, allowing absolute economy of movement. It also gives good stability of the picking hand and all strumming motion is done by the wrist, not at the elbow which offers less control.
Finally, being braced between the legs and the picking arm means the neck is stable and doesn't need any effort to position it form the fretting hand.
What engineers would call a very elegant combination of factors.
In my very first guitar lesson the instructor spent the entire lesson explaining the ergonomics of the position and nothing else, it's that important.
**See me in my avatar with my dreadnought. At 62 I now experience some fatigue between the shoulders after about half an hour with a dreadnought, but not with my shallower grand orchestra size guitar.