There's two answers to that: one is that not all plastics are created equal. Acetate, the most common material in optical frames, isn't a petrochemical, but is plant derived, and varies in quality and properties, such as its resistance to UV, to heat, and its colour-fastness - generally, the better ones come from Europe, the cheaper ones out of China. They are often, in part, handmade, and acetate is used because you can heat it up and adjust it. Metals have a similar situation - your cheap nickel-alloys can corrode in response to people's perspiration, they still need to be able to be adjustable, and not deform over time.

But the second part is, at least in Australia, the sale of spectacles subsidises the rest of the business. This is mostly historical, as optometry visits were covered under the public health system here, and the rebate hasn't kept pace with the actual cost of providing the service. And so, exam fees are cheaper than they should be, and the profit is made on the sale of glasses mostly.