This is being repeated over and over again on this page, but it's factually untrue… I've tasted dozens of kinds of wine grapes, and lots are tasty or at least as palatable as some of the blander table grapes – some are even widely recognized as such (muscat!). Also, even varieties that are not great for eating can often be used to produce some very nice juice – cabernet sauvignon, merlot, chardonnay are all used for high-end, monovarietal grape juices in France for instance.
"Eating" grapes are mostly not optimized for eating, they are optimized for shipping and lifetime at the supermarket. Wine grapes taste great, but don't travel well (and have bigger seeds, smaller fruit, etc.). American supermarkets sell mostly seedless grapes, which don't taste great, but are easy to eat and to give to kids.
Not in my experience. The best eating grapes are wine grapes by far.