The power level of a Fighter is strongly tied to character level. At lower levels they are fine, but a lot of their damage output assumes certain conditions (often being able to charge). Whereas caster gain not only more power with levels, but far more options; a well-built caster can get around nearly any obstacle, even if he needs to rest and change his spells first to do so. Fighters also have strong gear dependency, needing to spend most of their wealth on better +s in order have competitive attack and defense values against equal CR enemies. And then there is the lack of non-combat options, while casters have spells that can not only solve non-combat problems, but bypass large sections of plot.
But a Fighter still has better damage, you say? Though this is usually true, it isn't the sole metric of combat ability. A caster usually has multiple spells of a type we refer to as 'battlefield control' that can change the rules of the battlefield like Entangle or Black Tentacles or Solid Fog enabling a "Divide and Conquer" style of gameplay severely lowering the enemy threat levels. They also have 'Save or Lose' spells ranging from spells that flat out kill an enemy to those that hinder him to the point where he can do nothing. And finally there is buffs, able to increase a casters' capabilities without the necessity of gear, thus enabling the caster to purchase items that expand their versatility even further.
Many of us love the concept of the Fighter, but don't like how they were implemented in the system relative to other classes (Especially those with Vancian casting).
