Transitions and Transforms weren't available in 1999. Proper CSS animation with keyframes was first available in 2009 and even then browser support wasn't great -- lots of polyfills like 'moz-' and 'webkit-' were required.
jQueryUI was miles ahead in terms of delivering cross-browser compatible UI animation, so why bother with CSS3 animation which wouldn't work on IE8?
jQueryUI was miles ahead in terms of delivering cross-browser compatible UI animation, so why bother with CSS3 animation which wouldn't work on IE8?