This story actually sent me down a brief rabbit hole. If there is any science they put into it, it's psychology. It's all about treating them as badly as the non-shooting part of the job could ever be (and likely worse), and weeding them out, all while doing the traditional "break down to build up" crash course in traumatic teambuilding. They barely need the average number of graduates to be active SEALs, much less do they need the rest of the applicants to do any remotely similar work. Weeding them out through sheer misery is as good as any other way, though even then the Navy doesn't want them dying of Rhabdo. No, the Navy will be happier if you die from pneumonia brought on by your steroids and viagra (apparently the blood pressure effects help reduce swimming induced lung edema) helping you push your body until it literally breaks down.

Navy BUD/S in particular is a recruiting tool for the Navy. They dangle a glamorous prize in front of the boys of America, a prize that is quite disconnected from anything else the Navy does, and they therefore sign many of them straight out of civilian life for four-year contracts with only the promise that they'll be allowed to try out. Well over half of the applicants don't do any actually useful Navy stuff before going to BUD/S; for them it's their first "training" after basic recruit training. When 90% or whatever of them drop out, they "serve the needs of the Navy" without even the thin guarantees of an enlistment agreement because by letting them do their insomniac beach torture running for a week, the Navy has officially lived up to their end of the bargain. So you've got all these kids, many of whom are already high level athletes and often have higher test scores or even degrees, doing whatever the Navy wants them to. Even the ones who don't sign up for BUD/S can still get pulled into the recruiting office by the romance associated with Hollywood warriors.

Once they wash out, it (anecdotally) seems like about half of them rotate into something useful (seemingly split between brain-jobs like intelligence translator and kinda-cool jobs like underwater ordinance disposal), and the other half get made "undesignated seamen," your average sailors who are applying new paint or scraping old paint or heating up bagged chicken tenders that taste like paint, basically all the jobs that the Navy has trouble filling. One amusing reddit poster talked about how they'd be doing all these thigns on the "USS Neverdocks." It also seems like, regardless of the job they move onto, the general impression is that most of the dropouts will be professionally useless for several months, and only about half ever become truly productive sailors. But nobody knows for sure, because the Navy won't tell anyone.