I'm guessing. A chair usually has arm rests. If you sit on it, you are somewhat enclosed by the arm rests and are thus "in" it. I know, a sofa has arm rests, but if you sat in the middle, you would not be very enclosed by them. Note that we usually speak of a king or queen as sitting ON a throne, which is, of course, a fancy chair. I never said English was logical. Some phrases just appear by themselves and nobody knows exactly when or why.

You can use "on the chair", but "in the chair" is used for armchairs and the like. A person sitting in an armchair looks much more "in" than "on".

Sometimes English makes no sense, my chair is comfy and I sit in it. My sofa is wood, and to be honest, I never sit on it. Really my sofa is wood.