Sattelites are always falling to the ground, but they are also going sideways really fast. Imagine tossing a pebble off of a bridge onto a boat - if the sideways velocity is too high, the pebble will miss the boat and land in the water instead. Sattelites are like that. They are going sideways so fast that they miss the Earth entirely as they fall to the ground. If you want a sattelite to come back to earth, you need to get rid of that sideways velocity, which takes fuel.