Because this characteristic of iOS is an antiquity of when Apple needed to artificially slow the scrolling of the webpage in order to allow the webpage to render smoothly. It was one of the tricks used in order to maintain UI fluidity.
The current hardware is past the point where that is required but they have still maintained this.
This is totally erroneous. You really don't know what you're talking about. iOS renders everything as essentially a 2D OpenGL game to achieve the fluid manipulation across the board. That means in long lists like the iPod, to images (technically webpages are all manipulated as images). The physics of browser scrolling are not related to the fluidity/scrolling performance. It was a deliberate choice. The iPad has a different scrolling speed, apps can set their own inertias, and since iOS 4 (I think), the scrolling physics have been more responsive. The faster and longer you scroll now, the faster the browser scrolls.
Also, launch screens are not "tricks" to make things appear to load faster. You guys are amusing.