Because obviously, someone who happened to pick the right class should be able to break the campaign wide open, while someone who picked the "wrong" one should struggle mightily.
Yes, that's the fun of RPG. Picking a weaker class and still dominate in the end is more of a challange and therein lies the fun.
Some people enjoy being OP. Let them. I don't mind mages being ridicolously OP demigods, that's what they are supposed to be.
Imbalance works best when it's within a certain limit (so that at least most well played options are viable and none are game-breaking), kept in check, and easily remedied if things get too out of hand.
Get out of WHOSE hand? This is not a competitive multiplayer RPG, this is a single player game that can be played as a cooperative multiplayer game.
Really, fuck overpowered characters in a social game like D&D.
LoL what? As I said this is single player xD
Or could you explain how having someone basically do everything in a campaign while everyone else is rendered redundant and sitting on their haunches could at all be considered fun for the non-op characters?
You do realize that single-character players exist right? People who don't use companions and such? Yeah. Why should their fun be ruined in the name of balance? Pro tip: it shouldn't be.
Also your philosophy is exactly what leads to all characters being exactly the same apart from some cosmetics. If you want to know where balance leads, look at WoW MoP, where every class could heal themselves and pseudo-tank while dealing good damage. Basically balance always ends up meaning that everyone is the same with cosmetic differences like, oh I don't know, the races in 5th edition already are.
