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?
The fact that CSS3 was released in 1999 and such an app in 2023 is a cool HN-worthy post shows how easy, accessible/programmable/inspectable keyframe based animation got stuck with not much relief in sight. Flash dropped dead like a decade ago leaving a void, what remains is its husk as Adobe Animate, and droves of so-so frameworks and communities around them. Robust CSS/SVG based animation remained mostly an empty promise. You can find nice examples, sure, but it remains a niche craft.