UIWebView, (and android's webview) is on older implementations of webkit, and the gatekeepers of modern apps (Google included) have incentive to keep it that way.
They want devs to build apps for their own 'ecosystem' that are not interoperable on all platforms (eg: the web). They have disincentive to push for adoption of newer versions of browser tech as it encroaches on their respective babies (the OS').
UIWebView is precisely the same WebKit that powers MobileSafari, which is typically kept in sync with WebKit for the corresponding OS X release (so iOS 6.0 and Mac OS X 10.8 have roughly equivalent WebKits).
Pretty much the only difference between UIWebView and MobileSafari is the fact that MobileSafari runs JavaScript faster (by virtue of being allowed to mprotect pages, which almost no other app on the system, including 3rd-party apps, are allowed to do). But aside from a speed difference in JS execution, UIWebView is the same WebKit that's used in MobileSafari.
Not everybody runs Chrome.
and iOS and the rest: http://www.quirksmode.org/webkit_mobile.html
and good luck actually finding a sold version history of webkit on iOS, 30 minutes of googling have led me nowhere.