because you already have enough [censored] to kill an army with thats why.
So He has a Masters degree in Kicking ass. He's going to need it if he wants to stand a chance of besting a Dragon, even with others' help.
... he's not the only one. Cyrus the Wanderer, the Champion of Cyrodiil, the Nerevarine, and Hero of All Time all had PhDs in that field, so I'm not breaking lore.
Being a one-man-army isn't all that rare in Tamriel.
Due to the fantastic nature of the setting... armies have a tendency to be surprisingly incompetant when not supported by a PC.