Why is running on a treadmill easier than on actual ground?

Actually it depends on what you mean. Running on ground is "easier" in terms of running because of its harder surface, meaning more energy return. However it is "harder" on the body because of this hardness. On the other hand, treadmill is easier because of the lack of any wind resistance, and I have heard that when you land on a treadmill, you slow the belt down. And then the treadmill has to speed back up. So you really aren't going to belt speed the whole time, but just the "slowed" belt speed when you are in contact with it. I'm actually not totally sure that running on a treadmill is easier, however. The world record for running a marathon on the treadmill is well slower than even an olympic trials qualifier in the marathon in the US. And over a hundred people qualify on the roads going the same distance.