The truth is, the guy on the Golden Throne isn't really the Emperor at all. The Emperor, blessed be his name, was tired of all that fame. At the last moment, when flashes of light were flying around after Horus was killed, The Emperor quickly swapped bodies without anyone seeing. He stuck some dead legionnaire into his armour and he animated the corpse for a time with his psychic powers, before making the corpse 'die' on the throne.
The Emperor is now chilling out on a paradise world somewhere, pretending to be a waiter called Jose. He's much happier in his job, especially the lack of responsibility. He only has to remember drink orders, not run an entire galaxy. Also, his cocktails are amazing.