This is quite simply answered:
- Web views are easy to program. CoreText is not easy to program, even average CoreText usage involves lots of code.
- Web views are very fast. They have been optimized by lots of professionals. No easy to copy.
- In order to make CoreText fast you have to do a LOT of work. Building a performant CoreText view would require you to buffer, transform coordinates, etc. Nothing that needs to be done with web views.
- As core text ist rendering only, you'll have to implement stuff like scrolling (at least partially) on your own. Performance is the key here. Buffering and extreme optimization will be required unfortunately...
That's why noone does it. CoreText can be faster. But currently (iOS 4.x) this is a LOT of work. Nothing to be done in just a few hours. Especially if you're not a pro, leave your hands of CoreText for large texts for now. My opinion...
EDIT: Things get hairy if you want to embed images, etc. You need to pass all these things down to core text. Requires lots of fine tuning...