Quick fix for wizards.

[Beheld]

It is both actually. CR system assumes you play as 4 player, with distinguished roles.

This is incorrect on an explicit level and an implicit level. First, the game is not balanced on 4 players with different roles, the rules explain how ELs apply to parties of 1 and parties of 10. 4 Characters of the same "role" are balanced the same way as 4 parties of different "roles" because no part of the rules state that balance is predicated on "roles" at all.

Implicitly, you are wrong because you are trying to imply that those 4 roles have to be covered by different classes, but they really don't, any possible role in the game can be covered by the Cleric, and they clearly intended that, because you don't give a class Divine Power and Find Traps and Banishment if you don't want him to be able to do all those things.

The wizard in given example has just stated some items to boost his CL and casted spells. No class features(from PRC that advances wizard casting), feats, or any other essential items. Yet still can triumph over a CR 20 pitfiend, with a 40% chance outright killing him on the spot. CR system, as noted by Beheld, assumes he has a chance of losing of 50%, which as it seems, hardly comes into play. While, as a team game, you would need your teammates to cover your weaknesses, a wizard could simply say "Just tag along, I got this by myself".

1) Optimization matters, I can do the same thing with a Rogue and Fighter. I also explained how a Pit Fiend can kill the Wizard by having custom item choices that cover for his huge glaring weakness. Hell if he has constant Mindblank from an item, then the Wizard may never find him and he can just assail locations with summons and undead from safety.
2) Level 20 always has and always will be a bad example. The game becomes a fundamentally different game that breaks down at level 15. High level characters are not playing the same game at all, and they basically operate entirely based on the concept of dictating the terms of engagement. Level 20 is also never played. If you want to complain about how Wizards are broken, because they are too strong for the CR system, use an example that involves a Wizard from level 1-14. If you can't, then the argument you make based on a CR 20 is flawed.