'Why didn't the Vikings learn to fly?', in view of their undoubted skills in the sailing department. While it's true that they didn't actually come up with 'sail planes or hang gliders', the Vikings certainly spent a lot of time imagining flight, and indeed imagining the devices that might make it possible - their mythology is full of flight.
Freyja's feather suit (fjaðrhamr), is mentioned in Þrymskviða as a device which is borrowed by Loki in order to search among the giants for Thor's missing hammer. A similar falcon suit (valshamr), also owned by Freyja, is mentioned by Snorri in his Edda. This time Loki (again) is searching for the goddess Idunn of the golden apples, who has been abducted by the giant Thjassi. But things get complicated, because Thjassi, too, has his own flying suit, this time in the form of an eagle (arnarhamr).
Similarly, the maidens at the beginning of Völundarkviða flew in from the south wearing swan-feathers (svanfjaðrar). Later in the poem, Völundr himself rises into the air using something called fitjar, perhaps best imagined as flippers of some sort. He is however not really flying, just trying to raise himself up after the evil King Nidud had hamstrung him.
Others, too, have wanted the sailing Vikings to fly. A wonderful children's book, The Ship that Flew, by Nottingham author Hilda Lewis (1939), derives its central conceit from the god Freyr's magic ship Skidbladnir. According to Snorri's account, this ship can accommodate all the Æsir fully armed, and immediately raise a wind whenever the sail is hoisted, but can also be folded up into one's pocket when not required. In Snorri, the ship only sails, but Hilda Lewis imagined it flying and taking the children of her book on all sorts of magical adventures. I vaguely remember a TV adaptation of this some 15 or 20 years ago (has anyone got the details?) which rather boringly turned the central conveyance into a flying carpet, so much so that the story was hardly recognisable, yet I am sure it was the same story.