Seven has neve been a Starfleet officer before now. Even when working on Voyager she was constantly disobeying orders, talking back, doing things that would get a normal officer reprimanded or even kicked out.
She needs a few more years under her belt before she can be a Captain.
this is probably the exact reason she was placed under the command of a by the book hardass like Shaw. Janeway gave her lots of leeway. Shaw doesn’t.
As for Kirk, I doubt he jumped from Lieutenant to Captain. I’m sure he was at the very least a Lt. Commander/Commander and first officer somewhere before being handed the keys to a ship like the Enterprise. Then again, we don’t know. His career pre-Enterprise is largely unexplored.
depending on how much they plan on using him, some of the blanks may get filled in on SNW.