The difference between 'in' and 'on' is whether or not you are 'enclosed'. If you have something round you, you are seen as being 'in' the object. With chairs, it depends on how 'enclosing' the structure of the chair is. With armchairs and wheelchairs, you have arm rests around you, so you generally sit in armchairs and wheelchairs. This is different from stools and chairs that just have seats or seats and backs, which you sit on. With beds, it's much clearer: If you are lying on a bed, you are on top of the covers. If you are under the covers, you are in the bed. Also note that if you're there for a purpose (sleeping or with 'company'), you are 'in bed'.

I don't think it is completely illogical. "In" tends to go with being surrounded, being attached, being enclosed. Our kids our playing in the dirt. I hate playing soccer on (the) dirt. I'm cozy in my big chair. I'm sitting on my small chair. I lost my keys in the grass. I'm having a picnic on the grass. Let's eat in the kitchen. Let's eat on the balcony.

Well, I can't answer why, because there is no logic to it. But I can give you some examples. I am at my desk. I'm sitting in a chair. I am at my desk. I'm sitting on a chair. (Both are correct). My friend was sitting in his wheelchair when I arrived. (not on his wheelchair) I thought he would be in bed, because he had told me he was ill. I went to his bedroom first, and all I saw was his robe, which was on his bed. He was downstairs having breakfast on the balcony. (not in the balcony) Also, I'd like to add that a person sits: on a bench, on a stool, on a couch/sofa in bed (if under the blanket), on the bed (if not under the blanket), in the bathtub (if in the water), on the bathtub (if on the edge of the bathtub) in the bathroom, in an easy chair, at a desk, at a table on the toilet, on the floor, on the grass, on the tennis court, on the deck, on the roof in the garden, in the basement, in the attic, in the kitchen/office/living room, etc.

If you are enclosed, surrounded, then "in". Seems a balcony is more on a platform than in an eclosure. :) Was he on the enclosed balcony? Was he in the the enclosed part of the balcony? On a motorbike, in a car. On the train :( In the carriage of a train. On a ship, on a boat. Written in the newspaper. The cat is on the newspaper. In the news. On this website :)