Originally Posted by Child-of-the-Emperor
Another reason could possibly be the inherent purpose of the Adeptus Astartes. They are the protectors of humanity, not their rulers. They do not administer or lord over humanity, they use their considerable power and influence solely to protect humanity from the countless threats that they face in the harsh reality of the forty-first millennium. The Emperor consistently stated that the Imperium was not his (and as a result not his logical successors; Primarchs/Astartes), but belonged equally to every man. I see this as a logical reason as to why Astartes (or at least representatives of the entire Astartes brotherhood) don't sit on the council of High Lords.
Actually I think you hit the nail on the head right here!
I think a good analogy would be like God, Angels, & Humanity to the Emperor, Astartes, and Humanity. God created the angels to watch over humanity and protect them not to rule over them.