Adeptus Astartes, famously, did not involve themselves in politics. The reasoning behind that was something about humans getting to be ruled by regular humans, not transgened monstrosities bred for war.

This was a lie, of course, and a very flimsy one. The highest ruling body, the High Twelve, has been a testament to that lie - psykers from Astronomican and Astra Telepatica, abhuman strains from Navigator houses, bioengineered murder machines from Officio and Custodian Guard, an ascended form of Fabricator General - humans were ruled by things stranger and more alien than Astartes and nobody got to object that. The truth, as Toros assumed it to be, was hidden in plain sight all along. 

Astartes were children, brainwashed and thrown into the hellish meatgrinders to do what no ordinary man could do. They could - they did - conquer the Galaxy. They could not comprehend ruling it, having no frame of reference outside of the battlefield. With time and effort, they could reforge themselves into decent rulers (Imperium definitely seen worse), but that would mean stepping out of the comfort zone and assuming accountability over their decisions as rulers. Children were never good at doing that. Too bad that their Allfather was not there to gently guide them in that direction.

Which is why Astartes, the spoiled bloody cherubims of war, enjoyed playing politics without a care about the consequences of their choices. They were not supposed to hang around and see how their momentary choices shaped the future of the Imperium; they were allowed to walk away to another deployment. They involved themselves in politics at every step, just preferring to hover above menacingly and fly away once they are not in the mood - leaving somebody older, smarter, and, likely, more depressed, to pick up the pieces. Just like Adeptus Mechanicus and Custodian Guard were left to sort out that scorching crater left in the Imperial leadership circa M36.

Astartes make moves, and you react to them. Sometimes they even make moves you want them to make. At all times, they are a major pain in the ass.

She turned around to the armour-matron, glancing her up and down. You can't bargain with Astartes. She was no Astartes. And the very fact that she was here proved that she was already bargaining.

"Words matter not, choices do. I chose to appeal to the Angel of Death to avoid pointless bloodshed. Your master chose to leave me alive and sent you here, fully informed of my proposal. You chose to come here and negotiate my terms, can appreciate that, no time now. In sixty seconds, I will have to choose to resolve the problem less elegantly, some blood of good men needlessly spilled, my proposal terminated, you'll have to get the answer out of me... less elegantly. Choose wisely.", Secunda's voice had a cadence of metronome until interrupted by a soft scoff. "If it makes the choice easier for you, I may solemnly swear to get my tits out for lady Sarra. In about a week, of course, once the surgeons are done."