Why do Italians eat tomatoes?

The precise answer is because they were introduced to Italy following the discovery of America by the Spanish.

Naples at the time was under Spanish rule and the commercial ships brought this strange new fruit, or is it a vegetable, to the city.

It soon became popular and lent itself to early fusion cooking.

The zone of Mount Vesuvius and much of the Campania coast still provide Italy with most of it's tomato crop, although beware of illegal importation of tomato paste from China passed of as 'Made in Italy'.