1) It's a t.v. show.
2) Even though they're not travelling through time or (beyond 400 miles) space, there should be an infinite number of worlds where Earth never formed, the Big Bang never happened, or the Earth formed earlier or later and is in a different point of its revolution. Thus, you would end up in empty space and then die. It doesn't matter that they aren't travelling through time. Also, they aren't sliding to "different realities of the same world", they are sliding to "different realities" period. There is no particular reason that Earth is always there except that it works for the plot.
3) Quinn is a genius and would have thought of the odds of ending up in space. If a genius invents a Sliding device, knows that it is likely to land him somewhere where there is no planet, he would have immediately went to work building a safety into it where the portal would open near the largest mass (i.e. a planet) and specifically within a certain distance of the ground level. It may even check that there is breathable air. We know that it can control the location of the portal relative to the planet, otherwise the 400 mile issue wouldn't work, so it's conceivable that there is one aspect of the timer which keeps them within a certain distance of the ground and another aspect which keeps them in the same physical location. That second part was swapped, and now they only arrive within 400 miles. If the first part is intact, they would always land near the ground (and, if it was somehow calibrated to ground per se, with dirt below and air above, then would never land in the ocean either, or in worlds with no atmosphere). They might still land in a fire world, though. And hey, they did!
4) Return to point 1: IT IS A TV SHOW.
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