According to legend, the practitioners in the immortal world were insensitive and allowed the demon cultivators to massacre all the mortals in the local area, which caused the anger of heaven.
In anger, Master Tiandao sank the immortal world into the filthy lower world and ordered the immortal cultivators to enter the world and practice.
It's called "reinventing".
Since then, a vast number of "immortal workers" have poured into the secular world.
