The issue here isn't open source, its that google forbids newpipe to be in the play store, so there is a artificial market gap created by googles policies. These clones exist because google takes time to remove apps, such as newpipe clones.

The cloners are exploiting bureaucratic inefficiency, and they are providing a service the play store users, making forbidden software available to them, even if temporarily.

The real solution should be to get the app stores to be neutral, but thats not a fight any of us can win, I realize.