The food wasn't eaten during a formal seated gathering at a rectangular table, as shown in many religious art paintings, but with Jesus and his apostles reclining on floor cushions, as the Romans did at that time.
You asked us, "Why did ancient Greeks and Romans eat lying down?" Reclining and dining in ancient Greece started at least as early as the 7th century BCE. They ate lying down while others served them. It was a sign of power and luxury enjoyed by the elite.