For that matter, I know a number of folks who don't know what a web browser is either. It's just "The Internet", thanks to Microsoft's marketing, or "the Googles", thanks to Google's. When I used to read Google user feedback, it was amazing how many people thought that anything that happened online or offline due to a Google Search was Google's fault - we'd get feedback like "The plumber came 2 hours late...I want my money back!"
(You can install file browsers other than the default one on most OSes, true. But how many people actually do that? Even fewer than install Web browsers.)