Clerics have 5 spells per day per level plus a 6th spell (the domain spell), when Wizards get 4 (yes, you see correctly, Clerics can cast 50% more spells per day than Wizards
Clerics spend half their spells healing other party members. Many times out of combat. Being a walking, talking potion of cure (insert wounds here) is not fun. 3.5 Designer's recognized this and responded accordingly giving them more spells per day.
Many of the cleric' s spells are mandatory to counter many monster abilities. How many players would pick every spell correctly. "Hey Jozan I'm poisoned"....."Sorry Regdar I uh forgot to learn that one." "Aak, that vampire drained a level, can I get a cleric?" ....."You better make your fort save tommorow Regdar or that will be permanent, I can't help you"
Once again clerics spend half their spells healing other players. Without spontaneous casting clerics would be a boring class like they used to be.
In order to survive on the front lines, healing party members, buffing party members, fighting, The cleric needs higher HPs. If the wizard is constantly taking attacks, he is doing something wrong.
Clerics rely on melee ability. They are considered secondary fighters, that's always been the case.
Clerics need two good saves. If the wizard goes down, the cleric can heal/raise him. When the cleric goes down, TPKs tend to happen.
Once again....D&D is a roleplaying game. A Cleric's role is to be a front line combatant/healer, the wizards role is the exact opposite. Plus wizards in heavy armor are just kinda lame IMHO.
Wizards are proficient with better SPELLS & FEATS than clerics. A wizard that uses a weapon on a regular basis (past like level 4 or 5) is doing something wrong.
I've played and DMd PnP clerics, druids and wizards for years. Wizards are far more powerful than clerics at high levels. Clerics are good from level 1 however, where as a wizard's power takes time. Plus playing a wizard really well, is much harder than playing a cleric. The cleric's power is obvious and easily used by even beginer players. The wizard takes some experience and creativity to truly shine.
Druids can be tough though, depending on what the DM allows for feats and PRCs, but that is the case with just about any class. In the end though, D&D isn't about character balance, at least not the first 3 editions (who knows with 4E) It is about party balance and knowing your role...and making it fun.