I only found nice tomatoes here in Nanjing, I heard some farmers use human shit as fertilizer, it could be that?

Most likely reason is that it is too humid for them to grow outside of a greenhouse. They need to grow in a dryer area. It could also be too cold for them to grow properly there (most likely in the north), or they are grown in sandier dirt instead of nutrient rich soil. Or they are picked before they have reached maturity or... any number of things.
I doubt, however, that using human waste would make them colorless and tasteless. I've never tried it on my own tomatoes though.