I’ve been curious for a long time about this. The chuches I go to for confession is a room that inside has confession behind the screen with a kneeler and then one can go face to face but in a chair, why not kneelers for both or chairs for both? Just curious.
As far as I know it is just a custom. Penitents who prefer the screen typically prefer to kneel for confession. Penitents who prefer the face-to-face method typically prefer the chair as it enables a more conversational type of encounter.