@Arin-san
TL
R
MC is not a dumbass, he's just efficient.
Based on the information given to us in the first chapter, him farming for his own exp would take ungodly amount of time especially since he does not have any offensive skills (assuming that the whole island runs on videogame logic). Tho he Technically hold a sword and swing it around he probably would have a very difficult time since he does not have the swordsmanship skill.
Him being a porter is probably the most interesting way for him to gather exp by leeching to other players(assuming that the whole island runs on videogame logic). Because you got to keep in mind that his skill at start is just the goddess' blessing which gives him, by the looks of it, immortality. Yeah he could train to get the swordsmanship skill the way he got all those other "resist" skill but that would take so much time with a shit ton of hurt because he is "immortal" not invulnerable, or he would spend years of his life not gathering exp and jut hitting a dummy until he learn a basic skill.
So basically, him being a porter gave him an opportunity to be useful in the field and him being immortal makes him useful in combat by taking the aggro of the monsters effectively becoming a tank without spending a dime on equipment. and he's able to acquire useful defensive skills which could still benefit him even if he transition into a more active role in combat once he acquire a good offensive skill.