One reason:

"...performance of UIWebViews is less than in mobile Safari. This has a lot to do with the absence of the new Nitro JavaScript engine in UIWebViews, apparently for security reasons. I ran some tests on my iPhone 4 with iOS 5.1.1, the JavaScript benchmark Sunspider running in Mobile Safari was 3 x as fast as running in a native app with a UIWebView. Also, to communicate from the UIWebView to the native app, a JavaScript bridge is needed. This is tricky stuff, slow and not really thread safe."

http://blog.mobtest.com/2012/05/heres-why-the-facebook-ios-a...