Google is farming the “Build a site” out to others (Shopify, Squarespace, Weebly, and Wix), not offering it themselves.
Whether you’re a rookie or a veteran, our website building providers offer everything you’ll need to easily create a beautiful website. This is an optional service offered by Shopify, Squarespace, Weebly and Wix. For an additional cost, you’ll enjoy secure, reliable hosting, customizable web templates and powerful features like e-commerce, drag & drop creation and mobile site-management. No coding required!
Unlike Shopify, Squarespace, Weebly, and Wix, WordPress is software that you install on a hosting provider, not a stand-alone web service that you simply sign up for. What I mean by that is, it would be impossible for Google to farm out the site building to “WordPress” like they do to Squarespace without providing hosting themselves for a WordPress installation.
Google is essentially offering a domain registration with Google Apps, then directing you elsewhere for the website services (ironically kind of like how some hosting providers direct you to Google Apps for email instead of offering their own). So, considering this approach, I doubt Google can or will even consider offering hosting for WordPress installations (or any other web platform installations) anywhere in the near future.
Also, I do find it rather interesting that Blogger is never mentioned on the Google Domains site. Something for current Blogger users to ponder for sure.