You know, it's a common sight, to be sure - But not standard! Genies come in all shapes and sizes, genders and proportions. Usually it has a lot to do with genie self-image. Some of us just want that, to be sure. It makes us feel more ourselves, and we have the power to change our own appearance to match our ideal self, so why not? It's particularly nice when you have the capacity to defy gravity.
Another aspect of it is, and I'll confess this, managing Mistress/Master/Mastress expectations. We have the ability to gently tune our appearance to satisfy those expectations - in any direction - in order to keep up the genie business. Sometimes this is nice just so they can ease into their wishes. Somethings this is nice because we can distract them during their wishes (should they be nefarious).
That being said, yeah, people still wish for more all the time. Which can be fun!


