Yamamoto does hold his seat because he's the strongest, yes, but also because he's an admired leader and teacher.
If power was all it took, more people would have defected to Aizen
Same goes for the Hokage. Sure, strength is important, The kage has to be someone the others can look up to, and for ninjas skill and wit are just as important as power.
But most importantly, it has to be someone loyal, someone who can deal with other villages, and someone who cares about the good of the village.