Because Fantasia isn't something your local alchemist can produce nilly willy.

I love my Miqo'te, but sometimes, I want to be a Hyur. Or an Au Ra. On rare occasions when I'm angry even a Roe...for 5 minutes or so. So...I have such alts. I unfortunately can't equip them all with pretty glamours (Even when they reach the level, I'm poor and can barely afford a few glamour 'crystals' every now and again), but it's still nice to have them.