Medieval times had far less ability to travel. And when you did, you could find marvels and wonders quite close. Forest was so heavy in some regions that villages might be accessible only through the ocean. Men with no heads and their faces in their belly -- men with one only foot -- men with heads like dogs -- who could tell? (Though the dog-headed men appear to have been baboons. 0:)
Then the Age of Exploration -- with its technical advances -- interposed. And more and more technological advances. The steam-powered ship was an enormous advance. Not just because of speed. You could schedule your sea voyage instead of going to the port and hoping that a ship would happen by before your money ran out. (More importance for emigration than exploration, but had to help there, too.) Still a lot of blank spaces on the map, but in the era of steam power, they are generally defined spaces, with knowno reasons for difficutly in exploring them, and they were being checked off. (For instance, my mother hates the rhododendron hedges in Regencies, because rhododendrons were brought back in the Victorian era from Tibet -- which shows they did get to Tibet, even if several decades later, you could still situate a lost kingdom there.) Even the marvels tended to alter, lost dinosaur worlds, though that may have been taste.
And with modern technology -- well, in magitech situations ("Magic, Inc.", Operation Chaos) we generally have intrusions from other dimensions, and in urban fantasy, we generally have the hidden parallel world, because on earth there are no unexplored niches in the old style to adventure in.
Until you get off into space, of course. . . .
But the tech influences what sort of story flows easily from the setting.