@simon@simonwillison.netI think it's that webpages vary and break in ways that PDFs don't. They include images that may not display properly, they depend on libraries that can change or break, they render differently in different browsers and on different devices, etc.

A (basic) PDF is a promise of long-term consistency; every reader will see almost exactly the same content with almost exactly the same layout. It doesn't require ongoing maintenance to protect it from contextual changes, like a webpage does.