I find the most charitable interpretation that assumes good faith is they are test nets for potential future bitcoin features and apps, whose technical contributors are unwittingly being milked by fraudsters.
a surprisingly large proportion of the technical bitcoin founders I speak to have only very slight variations of essentially the same story: “I got into eth first, played around, had a great idea, tried to implement it, couldn’t figure out why it wasn’t working, in the meantime learned more about economics, finance, energy, and distributed systems” (i.e. the problems bitcoin actually solves), eventually saw the light, and here I am.”