The original copyright laws date from the 1700s; at the time the only thing being protected was text: stories, essays, reference volumes, etc. Basically, stuff for which there was no "source code" to conceal, the whole thing was right there on the page.

It's only been in the 20th century that we've increasingly seen classes of copyrightable works for which the source code dwarfs the final released product: music, digital visual arts, film, and software

To make matters even worse, the commercial interest in copyright doesn't care about any of this, because pirates only duplicate and distribute the end product anyway. So it's only the creative side wanting to remix and extend that is shut out by a lack of source escrow.