The ideas aren't mutually exclusive, and I've never seen an open source project support "curl | sh" without also supporting those methods.

Indeed, plenty of these scripts often act as a "what OS and packager do we have" mux. Just look at the source of this one, for example.

When you support an open source project at scale and/or with less savvy users, you come to see the benefit of "here, just f'ing slam this into your shell and we'll figure it out" installers. I know I have.