A variable aperture can easily make the aperture *smaller*.
But it cannot make the lens opening larger. You'd need a wider, bigger, heavier lens to achieve this. Which means a bigger camera hump on the phone.
So unless Apple wants to make the camera hump bigger, their only option is to add aperture blades that make the current lens have a _smaller_ aperture. Which means less light. And these tiny iphone sensors already struggle at f/1.7 or whatever. So making it smaller doesn't achieve much of anything.
The depth of field on a f/1.7 iphone lens is already more or less like f/11 on an 36x24mm format sensor, thus there's little need to have a smaller aperture. You could get a couple stops more depth of field, perhaps. Along with the option for a couple stops slower shutter speed in bright light. But the downside to a smaller aperture is increased diffraction, which reduces sharpness.
Plus having an adjustable aperture would increase mechanical complexity of the lens design, add a smidge of weight, probably also increase the cost by a few pennies per camera unit, and increase the liklihood of the camera malfunctioning were the aperture to jam.
