It's a 2.5 km (a bit less than 2 miles) underwater swim. Right now they've been without food for 10 days and are barely able to stand. At the moment they wouldn't be able to walk the distance, much less spend many hours (it was about four hours for the experienced divers, so more for the boys) swimming underwater in unpredictable currents with no visibility due to silt, so if they get confused and let go of the string for a moment in those hours then they die.

As one of the rescuers said, "When it starts raining the flow is so hard you can barely swim against it." - and that's a pro diver, imagine how would it be for a boy who didn't even know how to swim and thus doesn't have the relevant muscles trained. It's also narrow enough so that only one person can fit through, so you can't have the divers guide them by hand, only accompany them behind them.