I think the main reason to use plastic would be the potentially higher refractive index and better strength properties, which are also why plastic lenses are us

I think the main reason to use plastic would be the potentially higher refractive index and better strength properties, which are also why plastic lenses are used in spectacles (some glasses can have higher refractive indexes but still require heavier thicker lenses to avoid the risk of shattering). I would assume that glass lenses would be more likely to shatter if the phone were dropped. But it may really be that the plastic lenses can be manufactured to a higher quality for a reasonable cost.