Why aren't antennas placed at the ceiling of a microwave oven
It's not exactly an antenna.  Instead, it's a capacitive stub; a waveguide-probe (cavity-probe) driving a resonant &quot;circuit.&quot;
If you wanted the emitter in the ceiling, just turn the entire microwave oven on its side, so that the magnetron tube is above the oven cavity, rather than on the side.  (See the problem?  People want to buy wide, low-slung ovens, rather than ovens which resemble a miniature refrigerator!)
Antennas: driven cavity-resonance includes some weird physics beyond that of the conventional dipole antennas emitting EM waves into free space (the effects are similar to laser phenomena and atoms radiating into optical cavities.)   Unexpectedly, waveguide-probes can be physically short, yet still emit high wattage while showing the impedance of a much longer dipole antenna.  The short probe is driving into the synchronized fields of a waveguide, as well as driving against the strong fields of an existing standing-wave.  The tip of the magnetron tube can behave more like one plate of a capacitor, than like a quarter-wave dipole antenna.