While that's sometimes true, it isn't true in every situation and every context. Sometimes the designers use "target" in very specific ways.
Continuing the Fireball example, if you rule that objects are targets because they are affected by the spell, you create a contradiction. This is because, by the spell's text, all targets take full damage if they fail their saving throw, and half damage if they succeed their saving throw. But the language of the spell only permits creatures to make a saving throw, so objects can't succeed or fail, and thus can't take damage, which contradicts the statement in the spell that targets do take damage. Ergo, in the context of Fireball, objects are both affected by the spell and simultaneously can't be targets of the spell.