Think of getting one kind of gear as a power-up to your character. Getting multiples of them gives you a stronger power-up. They restrict it by character to prevent you from overgearing multiple jobs at the same time on tge same character, basically. Alts have the benefit of having getting loot every week multiple times, but having to constantly keep running the same dungeon over and over and having to relogin every time you want to switch jobs sounds a bit tedious.