I think the image of fireball as something nifty dates back to first edition AD&D, or possibly before--anyway, dating back to when low-level magic-users were complete schmucks, whose combat potential was about that of a glass of grape kool-aid.
When you hit 5th level and got your first 3rd level spell, it was a sign you'd arrived on the magic-user scene. This was the first spell with which you could be more combat-effective than some jerk with a bow and arrows.
Low-level magic-users in 1st. Ed. AD&D were complete putzes. You got one spell. One. No ability bonuses. You had a choice of three (3) weapons for melee combat, which didn't make much difference because you used the weakest combat chart. You had 4 or less hit points, because chances are you didn't have a 15 or better CON, and to top this all off, you needed 2500 exp to get to second level, while the thief only needed, what, 1200?
In short, you were a useless drag, kept around by the party because once you started getting these 3rd and 4th level spells you could hurt stuff that they couldn't.