Less than six months since it opened, Elon Musk’s Tesla Diner has the feel of a ghost town. Gone is the Optimus robot serving popcorn, gone are the carnivore-diet-inspired “Epic Bacon” strips, gone are the hours-long, hundred-person lines wrapped around the block. Even the restaurant’s all-star chef, Eric Greenspan, is gone. The Hollywood burger-and-fries shop seems like a shell of the bustling eatery it was when it opened in late July. On a balmy Friday afternoon in December, the parking lot for Tesla car charging was, at best, half full. Inside what the company describes as a “retro-futuristic” diner, a...