Because of convection, heat rises, and allows the non-toasting part of the frame, etc., to remain safe to the touch. It would be very bad to take an existing toaster and try to operate it on its side.
Plus, the simplest way to get even toasting is even heat on both sides of the bread (except when you do Bagels).
I can see an argument about hot air rising, leading to unevenness, or about hot air drying out the bread, but 'the toaster would get hot' doesn't make sense to me.
Regarding horizontal toasting, that is something I had in mind. I might hack my broken toaster to try this out.
Because of convection, heat rises, and allows the non-toasting part of the frame, etc., to remain safe to the touch. It would be very bad to take an existing toaster and try to operate it on its side.
Plus, the simplest way to get even toasting is even heat on both sides of the bread (except when you do Bagels).