Because there was no way to be sure that the Dragonborn wasn't just a secret agent for the stormcloaks or a reinforcement, really its not that bloody hard to figure out. >.>
Because DUDE, even after you were established to be THE Dragonborn, guards still say "Gotta thinking, maybe
I am
the Dragonborn"
It's not that bloody hard to see that your argument is flawed. Nobody knows who the Dragonborn is up until much later in the game, and by the time you may very well have killed thousands of Aldmeri soldiers, Thalmor Agents, Thalmor Justiciars, and steal thousands of horses. What have you
And here assaulting the Jarl only means a few days of prison sentence