[marssaxman]

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.