Because employers are subsidizing their employee's eye insurance, which is owned by Luxottica, and only usable at Luxottica's shops.

Ultimately even with insurance it is often cheaper to go outside of Luxottica's network, but many people don't even check and just assume that because they have reduced cost "insurance" that it will be cheaper to stay within Luxottica's spiderweb.

The way you solve this is to split up Luxottica via strong competition controls. Have one retail giant and one eye insurance company, not one monolith doing both.