Someone can maybe correct me if I'm wrong but I believe the FAA regulations omly require lifejackets if aircraft get more than a certain distance from the coast, which is far more than the glide-down distance, and that US carriers typically keep a small sub-fleet of so-equipped aircraft for "overwater" flights to Caribbean Islands etc. Otherwise it's the removable seat cushion, goodness knows how effective that is. This is why the Hudson ditching passengers didn't have lifejackets, and why Sully didn't call for lifejackets to be deployed in the final descent - they weren't on the aircraft. t must come as a considerable surprise for US visitors to Europe to see a lifejackets briefing on every overland flight.
When you look at actual ditchings over time, this seems a nonsense, as all in recent decades have been close to land, during approach or departure; I think the last open sea incident where lifejackets would have been mandatory may be the ONA DC9 ditching in 1970, getting on for half a century ago now.