I felt like yelling out "troll topic" but then I realized that would be quite distasteful. But I think the explanation lies in two things:
1. People expect a party with a little bit more than a cabal of wizards and/or priests as the adventurers. I mean, seeing four robed guys seems out of place when instead you have the Fighter, the Wizard, the Rogue and the Cleric in a (arguably) normal party.
2. Wizards are kind of weak in the start (from what I have seen and experienced). As for Clerics and Druids, I don't know. There's not much to say against them in the same context.
Either way, third reason that I just now got in my head out of nowhere was that it's also because DMs aren't cruel enough to the reality-warping Caster Classes who continue to oppress their lesser team-mates who lack the glory of spellcasting. Also, my first character in my first D&D game was a Half-Elf Cleric (of Pelor), though my second option would have been Paladin.