I think there's a very obvious reason for this. And it's akin to an electrician telling you to drive through a populated neighborhood to see his handywork -- a designer can show you their work because it's public-facing and visible for everyone. A developer, on the other hand, can only show you open source work, or personal work that they have ownership of.
It's much easier for a designer to do this than a developer, and I think for this reason, many companies don't expect a portfolio from developers. It's often a nice-to-have, but they often rely on other aspects (coding exercises, Stack Overflow/Github profiles) to evaluate a candidate.
It's not paid off just yet, but this may be merely because I've not cast it about much yet. I have been thinking that the results at present are a bit sparse and impersonal, and I don't really make any effort to 'pretty it up', preferring instead just to distribute a link to the GH preview page for it.
I'd be happy for any feedback/advice on the idea, as I'm starting to reconsider a revision now. Link is here: https://github.com/jarcane/resume.hsy