Why would he make you Jarl? As far as he's concerned you're a soldier, not a leader of people.
He's not making you Jarl for the same reason we don't give veterans high ranking political positions; they may have done enough to warrant such a reward, but they can't actually do the job. Someone who spends their time running around thieving, killing and hunting dragons would be worse than useless as a Jarl, when the people will ultimately rely on him.
History is filled with military minds who move on to become a political force, using both their intellect (necessitated by the nature of a leadership role) and the fame and respect gathered while campaigning to propel them into the political foray.
And it's really irrelevant, because the society upon which the Nord civilization is based revered one's personal strength as well as their ability to lead in addition to their wealth in order to determine succession. There is no real reason you can't, other then the simple answer that 'It wasn't programmed into the game.'
Sometimes the simple answer is the right one. There's really no need to delve into complex examination of why something wasn't included, because the truth is they couldn't include everything. I think at one point during the stormcloak path Ulfric says he has to elect a new Jarl for Windhelm, but he also says that he'll need to prepare for the impending Aldmeri invasion as well. Neither of these things actually happen, but that doesn't preclude their inclusion in mods or official DLC.
You do realize, since you never stay in one place for a long time (atleast most of us) you'd made a really, really [censored] Jarl. >_>
stew·ard1. a person who manages another's property or financial affairs; one who administers anything as the agent of another or others.
O.E. stiward, stigweard "house guardian," from stig "hall, pen" + weard "guard." Used after the Conquest as the equivalent of O.Fr. seneschal (q.v.). Meaning "overseer of workmen" is attested from c.1300. The sense of "officer on a ship in charge of provisions and meals" is first recorded mid-15c.; extended
to trains 1906. This was the title of a class of high officers of the state in early England and Scotland, hence meaning "one who manages affairs of an estate on behalf of his employer" (late 14c.). The Scottish form is reflected in Stewart, name of the royal house, from Walter (the) Steward, who married (1315) Marjorie de Bruce, daughter of King Robert. The terminal -t is a Scottish form (late 14c.). Stuart is a Fr. spelling, attested from 1429 and adopted by Mary, Queen of Scots.