The hamburger bun has sugar, dextrose and maltodextrin, etc in it. When I bake bread at home, I don't add those things. The ketchup has corn syrup. These things might be small but maybe they add up.
Not OP but when I make burgers at home I bake my own potato buns (not necessarily on the same day). No bakery around me makes soft savory buns and the prepacked store-bought ones have this weird synthetic and stale flavor.