Larger numbers make it arbitrarily difficult to crunch down specifics, and in general just looks really, really silly. Example from WoW, their tanks had over 1 million hp. It started to get out of control with individual gear pieces giving over 500 in primary stats alone, and hp pools look really silly when you start to hit the millions (and at the same time, starts to lose the impact of gearing when you hit the hundred-thousands). In retrospec, by the fourth expansion, Players had more hp than a boss designed to be killed by 40 people, and as a single player, was doing more dps than an entire 25-man raid.
As long as they keep the ilvls lineiar instead of sporadic (10-15 ilvl per tier), it'll be a while before we get to that point, but each expansion cut off is what's the issue when it comes to gearing (How long do you want the highest tier of raid gear to last in the new expansion's leveling process?)
In WoW's case, the highest piece of gear available in Catacylsm (that is, heroic difficulty of Dragon Soul)
is the same ilvl as a dungeon drop of a level 85 instance in Mists of Pandaria (and mind you that cataclysm was a level cap of 85). In FFXIV's case, FCoB gear would last you up until level 55. It's a tricky line to balance; Do you want to reward players for having cleared the highest tier of raiding before the epxansion hit (in consideraiton of everything, such as how accessible/nerfed/difficult the raid was, and the tomestone gear alongside it and how accessible that was), or make it in line so everyone has a fair starting point regardless of their previous raid progression?