Let me put it another way for you...
Your argument, if I am understanding you correctly, is that if it exists or makes sense for the setting, it should therefore should be an option to the player? Talk about a slippery slope! In the world of Tamriel there are gods, right? Following your own logic, since it exists within the setting, I should therefore be able to be a god. "I want to be a god who destroys the world on a metaphysical level. I should be able to form cults and have a chance at recruiting every NPC in the world to follow my cause toward darkness. I should also be able to be the Jarl of Whiterun and ignore every peasant NPC that comes asking for coinage. Or a mudcrab! They exist, I should have the option to play as one too..."
The list goes on.
A sound, lore fitting reason is given for you not being one of the gods. You don't start the game as one of them, and you aren't one. Though on the topic of becoming a god. . . well, there was that whole Shivering Isles thing.
For a powerful PC to become a cult leader, light or dark, makes perfect sense actually. I don't know how well the AI and engine could support such a thing, but it CERTAINLY would be a viable roleplaying option. It surprises me that you fail to realize this.
I don't see why you would not be able to become a Jarl, if you can muster the power to conquer their Hold and to maintain dominance.
As to the mudcrab bit. . . if there is a transfiguration spell that allows it, and you want to *cough* waste *cough* spend all of your time scuttling around as the pair of ragged claws mentioned in that T.S. Elliot poem, then I say have a wonderful time doing so.
That said, you are looking to hyperbolize. I am speaking of reasonable options, and two out of the four examples you gave don't really fall into that category. There is an enormous difference between not being able to become a god, and not being able to get two or three NPCs to follow your leadership as loyal companions.
If you are suggesting that it is fine for lots of things to be viable in the gameworld but entirely off limits to the player, maybe they should also remove the option to have swords, magic and guildmembership. And you DEFINITELY shouldn't be able to perform dragonshouts, as rare as they are supposed to be. Much more uncommon than Jarls and cult leaders.