I definitely see your point, it's basically just a flashier resume.
Typically web developers, front end, and full stack have a portfolio. Where most of the reasoning behind it is simply influence from social media where people mention it's a good way to stand out.
From the amount of research I've done trying to find portfolios from back end developers it seems like they don't share the ideology that it's a good thing to have.
I think it might boil down to the possibility that they typically don't have an option to include a "personal website" on a lot of their job applications, that's my reasoning for having a portfolio.
I think it's simpler than that: the profession of software development had already been around for many years by the time the web came along, and it never had any culture of portfolios. Most of what most devs do has no visual aspect. It makes sense that a culture of portfolio-making would arise within visually-focused specialties like frontend web development or game development, but the reason backend devs don't do that is probably just that devs in general don't do that.
