Not sure what you mean by "Google killed it". JPEG XL proposal was only submitted in 2018 and it got standardized in 2022. It has a lot of features which are not available in browsers yet, like HDR support (support for HDR photos in Chrome on Android was only added 8 months ago, Firefox doesn't support HDR in any shape at all), no browsers support 32 bits per component, there's no support for thermal data or volume data, etc. You can't just plug libjxl and call it a day, you have to rework your rendering pipeline to add all these features.

I'd argue that Google is actually working pretty hard on their pipeline to add missing features. Can't say the same about Mozilla, who can't even implement HDR for videos for over a decade now.