Sure, some of the web is read-only, but what about commenting, contact forms, forums, eCommerce or anything else that requires a proper server? That's a big advantage WP and other full CMS have over static site generators.
As for forums, those almost don't exist anymore, and contact forms and ecommerce are better off being handled by software designed to do that rather than some plugin shoehorned into an early-2000s PHP blogging platform.
For example, contact forms in Hubspot or Salesforce, and ecommerce with Shopify.
Edit to add: If someone's technical enough to be self-hosting Wordpress the open-source project, I would trust them to do a fine job at selecting a more focused alternative OSS project for those "actual server" needs.
Or if you're really making some big time e-commerce stuff, then you're better off with bespoke solutions than something hacked together with WordPress.