A lot of the tier 1's spell list is gamebreaking. Gate, Shapechange, Ice Assassin, Genesis, ect.
D&D 3.5 was designed with the idea that the Fighter would tank, the Rogue would use skills and Sneak Attack, the Cleric heals and Wizard blasts.
That style of gameplay doesn't cut it against many high CR opponents.
Alright, thanks.
It's a ton of things.
You can't have a well played tier 1 with tier 5 classes without the tier 5 classes being overshadowed, for example.
Not to mention the aforementioned spells with gamebreaking properties.
Gating in a powerful monster to smash a single encounter qualifiers.
Or using Planar Binding, at all.
There are higher OP games that can handle this sort of thing better, but many tables can't.
Wizards get stronger more quickly than Fighters do.
It's still part of the game and it's very broken.
Yeah, most people don't chain Gate Solars. But the game doesn't become literally unplayable if you do that, while the same is not true of Pun Pun.