This idea that the original authors of the source code don't have any right to that source code (they're not even listed as authors most of the time), seems like a contradiction. If copyright is so important, why is it so easily stripped from its authors in some fields?

It makes more sense when you understand that copyright was hardly ever about protecting authors, but about protecting the interests of the more powerful middlemen.

I think it fits in with the general idea of factories where you work for a salary and the work product you produce isn’t something you can take away. The problem with words or code is they are easy to copy, so one could easily make a copy and take it away. I guess that’s why they make you sign away the copyright

The factory analogy is pretty good. Profit doesn't really goes to the workers, it goes to the owners. And one critical property of capitalism is that owners and workers aren't the same people at all.

Having programmers sign off their copyright has the same effect as classical capitalism: it separates whoever owns the code from whoever works on it. The owners can then enjoy the full royalties, while the programmers are limited to their salaries —just like the factory workers.

Factory workers could walk away with the fruits of their labour, if only they owned the factory. Programmers could walk away with the fruits of their labour, if only they retained the rights to them. And in my opinion, they both should. The means of production should be owned by the workers, not the capitalists.

If on the other hand you agree with capitalism, it makes perfect sense to have programmers sign away their rights.