Because humans, and by extension, airline seats, are primarily vertical in nature. Whereas cars are primarily horizontal in nature. So the elongated side is ext

Because humans, and by extension, airline seats, are primarily vertical in nature. Whereas cars are primarily horizontal in nature. So the elongated side is extending in a different direction. In a parking lot, the shape, combined with the mechanics of parking, means that it helps to angle things. Though, you do lose one parking spot or so in the process. In a plane, the sides of the object (in this case the upright coffin sized space you fit into) are roughly the same, so angling will do little to help and is largely unneeded to "park" yourself. Ultimately though, airlines are unwilling to lose that one row if they did compensate for the morphing a reclined seat does for the shape of things.