Mostly because they arrive in mid-summer, when the California garden is looking so hot, dry and tired. Often, you've forgotten where they were planted because the leaves died back with the last rains in May. Then out they nakedly shoot and the garden is suddenly full of blousy, two-foot high pink trumpets.
I also love Amaryllis belladonna because 'Bare Naked Lady' was Ada's first compound word. And because they make the bumble bees happy.
The scent, though, is ordinary, a bit like liquid soap. And they are a huge pain to plan around. The leaves are vigorous, crowding out everything within a foot of the bulb. Then when they are gone, you have a bare patch that's not easy to fill so late in the growing year. One of the best bets for pairing, I've found, is the Santa Barbara daisy (Erigeron karvinskianu). It also needs little water and you can cut it back as the Amaryllis leaves sprout in the late fall. Then it will grow to cover the bare patch in the summer. When the bare naked stalks shoot out of the daisies they are tall enough to still make quite a show.