Why is prepared casting considered to always be superior to spontaneous?

Many times, I’ve seen the case made for a specific prepared caster class being superior to its spontaneous equivalent (e.g. Wizard vs Sorcerer, Cleric vs Favored Soul, anything vs its Uneathered Arcana spontaneous caster optional rule version) and said case is usually based on the spontaneous caster happening to be one level’s worth of spell progression behind the prepared version. However, in my experience, the wider community considers prepared casting to be always superior to spontaneous casting. Why is this? The specific cases alone aren’t proof of a general rule and all else being equal, spontaneous casting ought to be superior. What argument, if any, is so strong that it knocks down all of the spontaneous casters?