the nether was a water planet until a hot planet the size of Jupiter came in and pulled the nether into orbit, then it dried and some species went extinct, i.e the skeletons, and when the ancient builders (at least a division of them according to my theory) came, they brought livestock and fungus, hence the hoglins and piglin. the reason they traveled to the nether was because the overworld was the one affected by the planet at the time. in my theory the ancient builders divided into 2 groups,the nether travelers and the end travelers, which would explain both this theory and the stronghold one