You're either thinking of a tiny picture vs the whole picture (web devs vs programming in general) or you honestly have zero concept of what programming looks like outside of web apps.
To give some more pushback on this assertion... I've been developing software for 21 years now, and the only positions I had where there was negligible open-source software were in the first couple of years (in the automotive firmware space [OnStar], and a brief stint in some defense contracting). For the last 18-ish years--firmware, systems, enterprise, virtualization, demos, security--everything I've worked on has largely been based on open-source software, or the product was open-source itself. What I've never done professionally: Web app development.
Honestly, I think it would be hard to overstate the prevalence and impact of open-source software in the field of programming (and really, the entire tech field).