Fallout 3 - at best you are a gun for hire
Fallout NV - at best you are a LT for some faction
Skyrim - Again your a LT for some faction
Why wont they let us be high king, empress, or grand poobah?
Why must we always be the main part of the game but never the true leader/winner?
Because this is not Fable.
On a serious note, if becoming leader were the natural conclusion to the MQ it would ruin the possibility of going off and doing guild quests after its completion. In what world could the ruler or monarch sneak around assassinating people or stealing things? It just wouldn't make any sense.
If there were a way in which to ascend to becoming emperor or some similar position, it would have to change the entire game from that point onwards. No more idiots saying 'go fetch my mead' and no more guild quests sending you to do [censored] jobs. You should have different questlines to follow, which involve taking your personal guard or hunting party somewhere.