I was having trouble and wasting my time by converting file by file in command line. This script could help you too:

• This script works on Unix based systems such as MacOS and Linux.
• You'll need cwebp command available.

1. Download webp-convert-directory.sh;
2. Open terminal;
3. Access folder containing images;
4. Run sh [file_path]. You can drag and drop file to terminal to get full path:
   • Example: sh /Users/ricardo/Desktop/webp-convert-directory.sh
5. And voilà! All files converted!