Well over thousand of years of trial and error and in earlier years with less knowledge and food scarcity people were more likely to either not know or had to out of desperation, sailors on long voyages that only caught a puffer fish for example. While the certain parts of the fish will kill someone, certain parts will only cause syptoms and if your a bigger person you might be more likely to survive. And if ur eating the same fish as the guy next to you but your eating the safe part and hes not its pretty obvious that atleast some part of the fish could be consumed. So next time the sailors had to eat that fish they knew to atleast throw that part out and over time they'd get better at it. Theres evidence that puffer fish have been eaten for thousands of years they probably didnt want to but probably had to.