Soil composition, fertilizers, vegetable strains, and weather conditions all impact taste. Tomatoes from my garden made from heirloom seeds taste significantly better than best boy, beefsteak, or even tomatoes on the vine. I've had tomatoes from Italy, in Romania if all places, and can concur that even the produce exported tastes worlds better than what you can buy in the average US supermarket. I'd wager that the type of produce grown in the US is the culprit as it tends to be more important to have shelf stable and attractive produce than tasty produce. Stuff from my garden or the farmer's market is comparable to produce I've had in Europe.