Really? You are Courier #6. You agreed to deliver that package. Benny shot you in the head, dumped you in a shallow grave, and stole that package. If nothing else, you have keeping your word, pride in your work, revenge, wanting your paycheck (that you won't get until you deliver that package), and wanting to still have a job.
I'd hunt the cretin down just for trying to make me break my word.
Plus, have you looked at that suit he's wearing? He deserves to be shot.
This is a reason, yes. But it's not a given everyone would envision their character reacting in this way. Some people might just want to run and hide until they're sure no one cares about them anymore, but that's not much of a game.
To put it another way, in the other Fallouts, you begin adventuring because you don't have a choice. You get forced from your home one way or another, so you can either sit down and die or start walking. In this one, being a courier familiar with the surroundings doesn't allow for that clean break with the pre-story that kickstarts the main character's adventuring career, but nothing substantial takes its place. Ttracking down Benny is a long-term goal, just like finding the water chip or the GECK, and like the other Fallouts, you can ignore that goal if you choose. But if you do, there isn't anything to propel your character forward at the start, that would universally compel
any character to begin exploring the Wasteland. It would be quite logical for many characters to say the hell with it and set up shop as a Bighorn rancher in Goodsprings.
Of course, that's not an option. In this way it breaks the cardinal rule of storytelling- doing something that reminds your player (or reader or viewer) that this is a game (or book or movie) and pulls them away from simply experiencing the story being told. This ends up weakening the main character. You may think the MC is fine, and I won't argue that point; all I'm saying is that, no matter how good you think the MC's story is, if they had offered a compelling and immediate reason for adventuring, the character and the experience would have been stronger.