@blowdart: I once had an interview with a company whose new project was to build a shopping cart application. I politely asked why any of the approximately 1.7 billion existing shopping cart components weren't good enough. The answer was pretty much the same as you got. They also wanted to build a résumé site that allowed people to post videos. Apparently, when they asked if I had any questions, I wasn't supposed to actually ask any, regardless of how tactful I tried to be.
Keep in mind that this was a company that didn't have any developers on staff or any experience with building software. Their products were primarily breast pumps and dog training collars and they wanted to build and sell a shopping cart component.