Sunbeam has a 10-round duration (concentration) + blinding
For all practical purposes, 10 rounds is ridiculously longer than any reasonable combat, so this is going to last until the enemy dies from it. Or you lose concentration.
Lighting Bolt and Fireball, like Magic Missile, are too damn good for their level. That’s a deliberate design choice because these spells are iconic - they’re what wizards do! These spells are too good, so that they can tempt players to play iconic spellcasters.
D&D's designers have actually revealed to us the reason for fireball's ridiculous damage, and it has everything to do with the culture and narrative of D&D, not its mathematical balance. Fireball is one of D&D’s most iconic spells. The sheer power of fireball is central in many fun D&D stories, as reported by playtest information from the playtests that gave form to fifth edition D&D. The designers saw fireball (and to a lesser extent lightning bolt) as integral to the experience of playing a wizard in D&D, and chose to improve its destructive power to encourage players to learn that spell over other less iconic ones.