Every cell is defined by a lipid cell membrane, including the hundreds of thousands of cells that compose the tomato. At the very least they're there as a barrier, but there will also be some specialized lipids that are involved in cell signalling. There may even be some stored fats for energy reserves.

We typically eat to satiation, rather than consume constantly. In terms of lab animals, you see the former in rats, but mice will constantly nibble at their chow.

The main issue here is that very few humans have an ad libitum diet. It could just mean our animal housing practices are detrimental to their health.