The jet streams are shaped like a pancake, wide and thin. Often just a few thousand feet thick but miles wide and long. The balloon pilot must be highly skilled to stay within them. The liftoff point must be in a protected area to avoid problems with surface winds. Radio communication with military and civil aviation authorities is necessary to prevent mid-air collisions with airplanes in the vicinity.
Landing is by chance and always dangerous.
Details can be found in several of the balloon team websites listed below. Since it is almost impossible to get back to the exact point of the liftoff, a balloon that lands on or beyond the same degree of longitude but within a broad range of latitudes will be the winner and holder of the only remaining record unachieved in modern aviation. It's not the prize money. The endeavor often costs more than the prize. The answer, of course, is because it has not been done.