Because supermarkets buy from all over the world seasons as defined by what fruits and vegetables are freshly picked and available have been blurred to the point of almost being meaningless. You can buy almost any fruit at any time of year, now. Assuming you mean home grown fruit from the British Isles you'll probably only find last season's apples, which are perfectly delicious, available now, or anything that has been preserved in alcohol, jam or chutney from last year's crop. You'll be able to find South African strawberries, though. But they somehow never taste as good as when eaten in July!