Off the top of my head:

* It's easier to write without pulling in dependencies.

* Being simpler syntax means smaller page sizes.

* In theory, CSS animations can be faster.

* You don't have to worry about attaching listeners to dynamic content.

* Styling with JS violates Separation of Concerns.

* `prefers-reduced-motion` is only available in CSS, so JS has to run a CSS query anyway.

CSS animations run in the compositor thread, so they are isolated from jank due to concurrently running JS.

JS tends to be slower to load, parse and run than CSS.

Additionally, animations are often tightly linked to your page styles which are set in CSS. It’s easier to reason about them if they’re all in the same file and language instead of split across CSS and JS.

For performance reasons we always prefer HTML, then CSS, and only then JS as a last resort.

Another argument, this time for performance: JavaScript is single-threaded, so while you wait for your hyper-detailed spring animation to play out, the JS engine does literally nothing else.