Re: Game of Thrones (book/show) newbie (please no spoilers!)
Because he is goddamn talented moreso than he physically resembles the character in the books.
Much like Ned, Robert and most of the other adult leads, he is physically quite unlike his book counterpart due to being aged 15 years and the casting director being limited to people who actually exist. (Ned looks like Sean Bean because Sean Bean is a BAMF, resemblance be damned.)
"There is no reason why good cannot triumph as often as evil. The triumph of anything is a matter of organization. If there are such things as angels, I hope that they are organized along the lines of the Maffia." - Winston Niles Rumfoord.