Why Are UFO's Round?

As i have stated previously, it would be mad to not think that life has only developed on this single planet and not on any of the other billions of billions of other planets in the Universe and it isn't unfeasible to assume that at some point they have not only found us but come and had a look, it's exactly what we are doing.
Now three passenger jets have reported sightings of UFOs above Britain in December and confirmed by Gatwick Air Traffic who spotted six objects they could not identify on their radars before they suddenly disappeared.
The pilots each saw 'two flat, silver discs', flying within 100ft of their aircraft with which makes me wonder why are UFO's almost always round?
All of our aircraft are a bulky shape with wings or rotor blades so if the Aliens have the technology and know-how to come all this way to peek at us and they are using round disk shaped ships, why are we still making our aircraft and rockets the same old shape?
You don't need to be a rocket scientist to see that in terms of any aerodynamic advantage, the best shape is the one we are using so being circular doesn't give any real advantage but a round surface, with no edges would mean it could evade our radar although that theory is shot to bits earlier on in the post when the Air Traffic control radar picked up six of them.
Not being a rocket scientist is another disadvantage when you come to consider does a ship need to be aerodynamic once it is out of the planets atmosphere so is a spinning disc more streamline for travelling through nothing but in that direction lays nothing but a headache.
My own conclusion is that they travel the vast areas of interplanetary space in a rocket shaped craft and then when they get to our doorstep, they send out the reconnaissance ships which are small and round to avoid being detected by Earth radar.  
The other conclusion is that the Aliens may have an immense technological advantage but the concept of aerodynamics has gone over their little green heads.