Most Helpful Guy
I think that's because Herrmann and Mouch didn't want to be lieutenants. I see them well refuse to be because they don't want to get troubles and leave it to Casey and Severide.
By the way, the age has nothing to do with grade and responsability, older person don't always want to get more responsabilities.
Most Helpful Girl
I don't think either of them are interested. Some people are better leaders and some. are better followers. Plus Hermann seems like he's right under Casey anyway. He becomes in charge when Casey isn't there.