Why is there so few Imperial symbols ?

By "Imperial symbols", I mean symbols that represent the empire as a whole, and I basically see two of them constantly : the two-headed eagle, and the skull, with very few variants in their depictions.

Considering how extended and old the empire is, I was expecting more variety. For example, the burning sword of the emperor could be depicted to serve as a symbol, or the spear of Sanguinius, maybe some circle that would represent Terra, a pyramid-like figure that would represent the Throne, a star that would represent the Astronomican, and so on...

My guess is that it is purposefully limited for the tabletop game, to have the faction easily identified by players, but I'm still surprised in the overral arts outside of the tabletop game to not see more variety