The same is true re: Melee guys in my 16th level game. Honestly I have not seen that many spell casters over time use PrCs. A melee type just needs to have a +1 BAB per level to be effective...and frankly losing a point or 2 of BAB is not that devastating, as to hit bonuses accrue at a startlingly easy rate.
The same is not true for caster level. A Wiz, Cleric, or Druid can lose out of 3 levels and still get 9th level spells. Most spontaneous casters can only lose out on 2 levels.
Is a PrC like Argent Savant even worth losing 1 spell casting level?
Some classes like Abjurant Champion are just too powerful, if you are playing an elf, why not take the class, it is win/win...but outside of a few clearly optimal choices...most spell casting PrC fit a niche, or add an extra burden to consider in qualifying to the already burned player of a spell caster.
Even something like the Master Specialist is not a given, in my opinion. A player has to decide to play a specialist wizard, which is not an easy decision for most people, and while the PrC gives out nice benefits, you have to forgo the Bonus feats, which are quite powerful.
The Wizard feat cycle of 3,5,6 is pretty defining at low level, and a nice boost. Getting Skill Focus: Spellcraft and a minor power is not always optimal. Moreover, the proliferation of Specialist Spontaneous Caster Classes like Warmage, Dread Necromancer, and Beguiler, makes Evocation, Illusion, and Necromancy specialists harder sells, and PrCs in some cases harder sells.
Very few spell casting PrC are going to give the skill points of a Beguiler, or the powers and Advanced Learning Options of a Dread Necromancer. The Warmage has some PrC options, but in many cases, is going to go down to a D4 hp when using a PrC.
Druids almost never go into a PrC,(why bother to), Clerics seem to gravitate towards RSoP, which frankly is as much a boon to the DM as it is to the players.