The Windows "Photos" app doesn't support webp, at least not without an extension, so it doesn't associate the file extension with the app. Chrome can display these photos, so it registers the webp file extension.

That's it - webp isn't treated as a first-class picture file format even on Windows. You can hardly expect Apple being more progressive this way.

https://www.irfanview.com/plugins.htm#UpdatedPlugins :)

Or https://sourceforge.net/projects/jpegview/ - slightly less (yet sufficiently) powerful but looks better.