It is not sweet, like most fruits. That is why rhubarb (a sour vegetable that is much like celery) is used much as a fruit, in pies. Tomatoes taste less fruit.
Try a diced tomato with sugar sprinkled on it. I've had it as a quick dessert since childhood.
people do just eat a tomato sometimes ive done it and seen it. i think why it may be less common in some places is because for many people its nearly always seen as a salad item or in a sandwich or part of a meal. i find they dont have the right kind of sweet taste to be taken as a dessert. i feel the same about grapefrits too as there so bitter.
They are predators in the fruits' world:)