In Fallout 1, the Vault Dweller was a wasteland-wandering hero with a serious super mutant death wish.
In Fallout 2, the Chosen One was a tribal savior who saved Vault 13's inhabitants along with Arroyo's.
In Fallout 3, the Lone Wanderer was a Vault 101 reject with a need to finish his father's work.
In Fallout 4, the Sole Survivor was an ex-military vet who got frozen for 200 years and then spat out of Vault 111 like a wad of gum, who then hunted his infant son across the post-nuclear Boston at all costs.
But in Fallout: New Vegas, the Courier is a vengeful messenger who gets shot in the head twice and recovers, and then hunts the man who did it across the Mojave until he plants a bullet firmly in his skull. After that, he kills Mr. House, benefactor of New Vegas, and then pushes both the NCR and Caesar's Legion out of the Mojave at the Second Battle for Hoover Dam, making New Vegas an independent power in the post-War American Southwest.
Now, don't you see a difference?
I see the difference. All the others you are a valt dweller that has a sole purpose and something meaningful. But as the courier you are just some random dude with the miafourton of the platinum chip making you go out on revenge and eventually making or breaking the Mojave