The run-of-the-mill tomatoes you see in American grocery stores are usually hot-house tomatoes and sometimes Roma. They are picked green and "ripened" in transit/cold-storage with artificial ethylene. If ever you had to ripen a peach or an avocado, someone might've told you to put it in a bag with a banana or an onion. That's cuz ethylene is a naturally occurring chemical produced by fruit/veggies that are ripening, but because Americans value looks over flavor/texture (I'm looking at you, red delicious apples), the tomatoes are red on the outside, but extremely mealy and flavorless on the inside.