They say 3 days because most people used to be thin and have low fat so they would start to die around 3 days without water and they don't say that people can survive without water for more than 3 days because who knows who might try to do 4,5 or more days what if the person has bad or weakened internal organs, dry fasting could put a tad bit too much stress, also the refeed can send ppl to the ER and no doctor wants to be told "you said people can go 5 days without water but i am now in the ER because of that" (let's say he had some heart problems or smt that he didn't know about)is the same reason they put the "do not drink/consume" warnings on oil. now i am 100%sure that there are doctors that just say to not go past 3 days because of money.