From Melbourne, @SamBam what do think is the reason?
My own personal observation, growing both Roma and large Beaf steak variety tomatoes in a home plot, is it's a combination of cultivar and heat. I agree on variety chosen. The commercial varieties are combinations of size, colour, season and preservation. Not necessarily taste. So it maybe the sugar content.
Leave the tomatoes in that sun, pick them when they are ripe and the taste is there. Back at the supermarket, picked early and cooled... not so much flavour.
I only saw zucchini for sale once in Japan and, for what is nearly a prolific weed -- can hardly give away all zucchinis if you have one plant, the zucchini cost like $10 a piece so I refused to buy them.
The most commonly sold variety of eggplant in Japan is the Italian type which is hardly sold in the US and naturally smaller than the US standard regular / classic type. So not really comparable.
Cucumber variety is also different. The US normally sells straight 8 type. The Japan variety is, I think, a Japanese cucumber and a different type. So not comparable.
Out of season? We buy them for 100 yen, which is still more than free, but they they taste better than the huge ones I've had in the US.
Tomatoes tend to be beefsteak, so not very good for sauces, but big and nice in salads, or mini tomatoes.
Considering they aren't my favorite, anything above a yen is too much.
I mean, in our defense, if we actually ate mostly or only what was in season locally, we would mostly live on squash, beans, bread, and potatoes. Real, tasty vegetables are just plain difficult to grow across much of the country, and shipping from the places with easy growing to the places with difficult growing brings in all the tradeoffs of trucked produce.
Coming from Italy, I'm always amazed how large US tomatoes/zucchini/eggplants/etc are, and those big, watery vegetables are almost always tasteless in comparison.
I think it's a combination of selecting varieties that produce bigger fruit, as well as waiting longer to pick, so as to produce the largest (most water-heavy) produce.