The real reason is that after level 50 (the highest level needed to beat the game) your characters don't really get many stat upgrades beyond that. My GBC copy of DQIII has a glitch that maxed out my main character after fighting some slimes in the mini-game board and I noticed after level 50 he stopped gaining stats, like every 5 to 8 levels he would actually gain a stat bonus but that still is only about 8 or 10 minor additions. I think VIII is the first game in the series to actually make it worth your while to grind to godhood and even then I don't think its worth it.