The first oozes likely dripped off of the back of Jubilex and just slithered on from there. The vast majority of them don't think in a way that humanoids would recognise. Instead, they exist to consume and consume to exist. With will pull themselves towards food, engulf it, and devour it. There is no morality or concern. There's not even an impression of individuality. Two oozes of the same kind next to each other might meld and become one. A single ooze might be split into several smaller ones.