If fresh lobster is cheaper than ever in Maine, then why are lobster rolls in New York City so expensive? Authors from the Atlantic to The New Yorker seem sufficiently puzzled by this to posit various theory related to the elaborate psychology of pricing.
Here's another theory. Lobster rolls in New York City are expensive because they are not filled with uncooked live lobsters on a dock in Maine. Someone has to buy those lobsters, ship them to New York, cook them, take the meat out of the shell, and then place the dressed meat in a split-side toasted New England–style hot-dog bun. The fact that "the delicious lobster roll you're ordering on Seamless right now costs you around seven to 12 times the amount that lobster meat is actually going for" simply goes to show that you are paying for the shipping and the labor and the cost of the building rather than for the uncooked lobster meat.
If you pay attention, you may notice that this is generally how all restaurants work. A vegetarian burrito at Chipotle has approximately $0 worth of food in it, but they still charge you money for it since assembling the burrito requires Chipotle's paid staff to do work and if you sit down and eat it you're taking up valuable space in the restaurant.
Something you may have noticed in the past few years is that there are way more lobster rolls for sale in New York and Washington, D.C., than there were 10 years ago. I would speculate that this is not unrelated to the boom in lobster supply that's created cheap local ingredients. If live lobsters weren't cheap to buy in Maine, then the entire concept of Luke's Lobster (and similar places) wouldn't make sense. The only lobsters transported out of their northern New England homeland would be destined for fancy plates at expensive restaurants. But with Maine experiencing a lobster glut, suddenly casual fast-food-style lobster rolls are proliferating out of region.