Celebrim
Legend
To further compound the problem of spellcaster domination at the higher levels, most gaming groups, were no exception, start at higher level making it easier for the squishy spellcasters to exist.
I think I'm only beginning to understand just how common it is for most campaigns to start at a high level. I'm fairly shocked by campaigns that start at 3rd level (I'd only previously encountered one). I've been playing nearly a year in my current campaign, and we've just got everyone to 3rd.
For me, much of the attraction of D&D is how much the game changes as your character advances in level. You start out grubbing in the mud, barely more compotent than the farmers and maybe not even that. You end up a demigod.
In fact the higher level you start the less multicasting you will probably see, as the necessity for such decreases with every level your campaign starts beyond 1st exponentially beyond 8th. As a result more pure straight, single classed spellcaster classes are selected making them gain access to even more powerful spells and having more of them to cast.
By late 3.X even that wasn't true. Most spellcasters had spellcaster PrCs that offered large benefits with only small drawbacks. In some cases there were Druid PrC's that offered a strictly better druid and Cleric PrC's that let you be a strictly better cleric.
This will also increase the spellcaster demographic, making them more numerous, increasing the amount of players opting for spellslingers. So we, the gamers, are guilty of making it worse, widening the gap between them into a cavern!
This is certainly true in some cases. Wizards and sorcerers in particular have an extremely difficult time surviving in a low level campaign that has relatively low access to magic (no magic stores). Without belts of constitution and bracers of defence being readily purchasable, and low level wizard is very squishy indeed. Faced with the prospect of attempting to slog through 8 or 9 levels with a powerful but fragile glass cannon, many players will opt instead for an immediately powerful and more durable melee build.
However, none of that applies to the Druid or Cleric. Those classes have a high amount of survivability at low levels, combined with tremendous broken upside at higher levels. You start out as a support character buffing and healing the front line. But, you quickly are going to be able to take your place on the front line if you want to. Whereas the break even point for the arcane casters is about 9th level, the divine casters should be easily holding their own by 7th level. And keep in mind, a lot of this depends on access to magic items as well. If my Wizard can end up with bracers of defence, a ring of protection, a belt of constitution, and a wand of fireballs by 5th level, things are really looking up. I may already be the parties primary damage dealer. Assuming that I'm not facing an old school Gygaxian RBDM, and it's clear that I'm not, I'll probably dominate the game from there on out.