Why doesn't the module fill the meta keyword tag?

The module generates titles and meta descriptions; it does not populate the <meta name="keywords"> tag.

Google states that it does not use this tag and that it has no effect on indexing or ranking in Google Search. You therefore do not need to fill it in for that search engine.

Focus on relevant titles, page-specific descriptions and useful content. The keywords tag is separate from keywords used naturally in your content and from image alternative text.

Source: Google’s meta tag documentation.