because somewhere along the line writers got the silly idea everyone wants to be in charge of something.
Take Skyrim for example. There is not a faction that does not at the end put you in a position of power and leadership. And funny thing is it doesn't matter as people beneath you keep giving you mundane quests over and over.
Same with Fallout 4 factions. "Oh your a badass getting a courser chip! I got a job for you. Put up a Mila?" Or "Hi, we just met but you seem nice. How about being General of the minute met while I dispatch you all over like a common soldier?"
Its all in the writing.
I'd be happy being able to complete a faction side without having to become its leader. After all what if I was just a helpful SS passing through?
I feel the same, thats why I hope they don't make you leader of the BoS in a DLC. Nothing wrong with being an underling.
Btw, another settlement needs your help, General.
I find the only way I can make Preston feel less like my boss is if I take him along as my companion ("come here" "tweet" "over here" etc), but that would probably annoy me even more...