Just tested with my local Mamp Pro (PHP 7.4.12), and the webp version is generated correctly in Kirby 3.6.
Hm, while the PHP version is slightly different, I would expect this to work for you as well (to be 100% sure, check if phpinfo() confirms that webp is enabled.
So as it turns out 7.4.2 (with MAMP pro) doesn’t support webp per default, and I’m not sure how to enable that (yet).
MAMP Pro wants me to upgrade to be able to use anything up from 7.4.2. I have little need for all the bells and whistles so I installed php 7.4.23 (with homebrew) and everything is ok.
As @texnixe mentioned, ImageMagick is faster, and has more features - eg., the latest version already supports AVIF, while the latest GD still doesn’t. So, if you use ImageMagick you can probably also output your images as AVIF, as well as WebP.