Also to stimulate the economy and give an actual use to it and motivation to SB the combat stuff.
But here's the thing about the animus->novus and nexus->zodiac stages: Many steps can be done in advance. You can start gathering materia pretty much from day 1.
The Alexandrite is a pain, yes, but there are no less than 4 ways to get it: Trading soldiery for maps, farming FATEs, doing hunts, and the daily quest.
It's actually the closest to epic weapons from other games that exists within this game.
Moreover, SBing is actually quite profitable as well. Since you don't need more than 3 types of materia for you relic (at least, you shouldn't) the rest are all gravy. Sure it sucks when an elemental materia pops out, but if you get a spell speed IV you didn't need, that's money in the bank.
The novus rewards steady progress and perseverance, setting forward what you need but not requiring any single route to do so.