Because they want people to use the seasonal items, rather than just stuffing them in storage right away to gather dust.

I sort of agree, but if they want us to be able to enjoy collecting items, they're really going to have to rethink a lot of things; even with FOUR retainers, space is at a premium for anyone who crafts or enjoys changing up their glamour reasonably often. They need to streamline things; fewer materials to produce 1 crafted item, and fewer materials that are good for only one thing and yet drop in crazy abundance (Coeurl skins, I'm looking at you!).