The framework is fundamental for all the rest of the stuff. You transfer it before the majority of the site's content. The faster it arrives, the faster your site is usable. It's also always in use. A badly sized image can cost a few MB, but that's a trivially addressable problem on one page of your site. The framework being 10x its required size is a huge development hurdle to overcome.

Your local mom-n-pop laundry place's website probably doesn't need these types of efficiencies, but if you're running a large ecommerce site, time-to-interactive is an important metric which can cost a lot of money.