Because you spend many hours building a complicated rocket with pretty much everything - from a little robotic rover to a freaking permanent colony for five - testing it and watching it explode over and over again. Once you get it flying, you set a course to a far away planet and spend couple of hours just waiting, watching while paint dries, beard grows and worlds turn. And once you get there, you begin descending. Careful not to spin the whole thing over. Not to go in too fast. Not in too low angle. Then the last drop of fuel burns out about 50 meters above surface. Your rocket hits the surface about 142 m/s Rinse and repeat. You tell yourself you can quit any time. You just don't want to.