why is it that wizards have the reputation of using mostly fireball and only fireball and nothing but fireball, when both fiend warlock and sorcerers are the only two classes who could reasonably devote all their resources to fireball without having lower level spell slots left, and the ones with the most fireballs per day (five for sorcerer at level 5 if you use all spell slots and sorcery points, two plus twice the number of short rests you take for fiend warlock)? Why is it not the classes with rather few known spells in their arsenal who have this reputation and instead the class with an very fragile, flammable item that stores their ability to cast most of their spells?

Fireball punches above its weight so most wizards choose it whether they're focused on offense or not. It's got huge range, covers a huge area and deals above-average damage for its level.

Before 3e, there were no Sorcerers or Warlocks. But Wizards casting Fireball have been a thing since 1st edition.

Cause FIREBALL goes BOOM!!!

FIREBALL!!!!!

Fireball is OP. It does more damage than any other spell the same level in a better area. 

The best instant damage dealer 2nd level wizard spell is Scorching Ray, which does 6d6 damage if you hit with all of the bolts against one target. Fireball does 8d6 to everyone in a 20 foot radius sphere.