One thing to also remember too is that 3e was a sudden shift from the 2e mindset.
In 2e, kits and other customizations existed, but they were suggested for concepts from the role-playing perspective, or the "story" side of things. In fact, I forget if its the PHB or DMG, but the 2nd Edition book there's a section saying that people SHOULD NOT Min/Max because it's not what the game is about. They discouraged people from playing that way. People who played that way were dismissed as "munchkin". About the biggest "build" was specialty priests, and that was more based on character and deity not tactics.
In 3e, Min/Maxing, multiclassing, prestige classes, all of that was encouraged. Part of the design of 3e was to make sure people remembered it was a game, rolling dice, hack and slash, etc. So you now shifted from de-emphasizing that to whole "min/max" articles in Dragon. (I believe WoTC recognized that many people played CRPGs that way and many table-top players played that way).
So, while customization started a long time ago, it became a bit more common in 3e based on the new paradigm.