Why are radiators not placed near the ceiling? What are the advantages of underfloor heating?

Because heating is associated with such a physical phenomenon as convection of air masses. This is when the air warmed up from the batteries rises up, and the cooled air goes down, where it heats up and rises up, thus circulation and uniform heating of the air mass in the room occurs. The batteries near the ceiling will only heat the ceiling, and all the air under them will be cold. The batteries, even those located near the floor, do not heat the floor, but heat the air, and the floor remains cold. In order to be able to walk on the floor without shoes in winter and not be afraid of catching a cold, a warm floor is installed.