There is a story told of mcdonalds and celery. Celery for those not familiar is something of a flavour enhancer. Their test kitchen produced something very tasty using it. However the story goes that they would need the world's entire supply and then some to distribute it in enough quantities for their "restaurants".
Something similar would likely have happened here and if they had a supply locked down (and somehow the seed banks around the world fell short, see
https://www.seedvault.no/ for the Svalbard Seed vault, legacy stuff was also not an option) they could probably make more selling it on than just adding it to their cheap burgers as basically a throwaway ingredient.
For myself. Lettuce (iceberg especially but cos, lamb's, Chinese and more are not much better) is basically a filler vegetable that is water with an almost tasteless* wrapper that I eat on very very rare occasion, though it is not like I have to choke it down as and when. Far more interesting vegetables exist.
*I did actually try a lettuce leaf in my dehydrator as one of the many things. Makes it a somewhat concentrated taste of lettuce actually.