I spent a few years in the eyewear industry.
Offhand, it seems like this stuff isn't that difficult to make. After all, it's just frames made out of plastic, right? But it is at this point we get a glimpse into what it takes to manufacture products.
The basic design of glasses - like any accessory - is a skill in itself! We look at the shape, the materials, used, the colors, the hinges, the nose-pads, and the aesthetic of frames. Many of us can look at a pair of glasses and "know" that they are styled for men or women, and different frame shapes and colors are better suited for different face shapes and skin tones.
This is all to say that, even before we start manufacturing, one has to find talent who understands all of these aspects of accessory design in order to produce a great product. Parallels to software engineering suggest themselves.
When it comes to manufacturing, most all of it is done outside the US in China. Physical manufacturing is difficult. There is a years-long iterative process of choosing materials, manufacturing samples, working with the vendors, and ensuring quality. Maybe you are importing acetate from another country. Oh, and it is February - you planned for Chinese New Year, right? Import these frames into the US and let them sit in customs for a month. Etc. Physical manufacturing is hard.
What if you're making eyewear for another brand? Now you have to design things which are consistent with that brand's aesthetic.
Then you have to find an optical lab to fulfill prescriptions. A lab which is reliable, does not ship products with scratches on lenses, and has strong inventory controls. Optical labs, incidentally, are really expensive operations to run because the machinery is so specialized.
And then there is the shipping, logistics, and customer service pipeline.
At a unit level, it is true that glasses are pretty cheap to make. In that sense, because it is easy to get into the market - which is relatively unregulated compared to, say, payment processing - you deal with a lot of unscrupulous businesses and middlemen trying to peddle bad-quality goods.
Eyewear may have recurring customers, but perhaps not so much brand loyalty. Many customers want to physically try on frames before buying, which increases customer acquisition costs (either via home-try-on programs or physical showrooms.)
Companies like Luxottica, I have heard, sell their eyewear to mom-and-pop optometrists and use sales data to figure out the size of the market. Then they plop down with a Lens Crafters.
This is all to say... it is a difficult business, there are many competitors, it is hard to wade through the crap, it is hard to find good talent, and things like aesthetics and brand recognition are important - and take time to develop. Not as easy as it seems!