> I really don't get why people don't just declare their work public domain.
Because in many jurisdictions, this is not a thing that can be done. It is not in the author's power. If you want people in those jurisdictions to be able to use your software, you need to provide some sort of license even if you consider the work to be public domain.
Even in jurisdictions when it is possible to do this, it's understandable that hobbyists would still want the protections of the "Don't use my name" and "Don't sue me" parts of 3-clause BSD at the least.
As for GPL, people choose that because they have different priorities than you. For an obvious and topical example, see Richard Stallman. He believes that nonfree software is actively harmful, and therefore takes measures to avoid accidentally supporting its development. You may not agree with this viewpoint, but he is clearly sincere in holding it, and it is his right.
Because in many jurisdictions, this is not a thing that can be done. It is not in the author's power. If you want people in those jurisdictions to be able to use your software, you need to provide some sort of license even if you consider the work to be public domain.
Even in jurisdictions when it is possible to do this, it's understandable that hobbyists would still want the protections of the "Don't use my name" and "Don't sue me" parts of 3-clause BSD at the least.
As for GPL, people choose that because they have different priorities than you. For an obvious and topical example, see Richard Stallman. He believes that nonfree software is actively harmful, and therefore takes measures to avoid accidentally supporting its development. You may not agree with this viewpoint, but he is clearly sincere in holding it, and it is his right.