With the Japanese love of convenience, why do they still sell such exceedingly old-fashioned and hard work versions of corkscrews and can openers?

When they are combined, they do have a certain convenience and, perhaps more relevant, compactness. Alternatively, it could be a case of borrowings from the west reflecting the level of technology of the time it was borrowed- like the hightech Japanese toilets reflecting the technology that was available at the (much later) time at which they became standard. In this case, perhaps by the time the market would have been ready to trade up to a classier or easier to use version, most cans already had tab pulls. As people don’t often entertain at home, there are also few chances to show off your Philippe Starck corkscrew or electric tin opener