Those built-ins that you are talking about are not as well rounded as a dedicated framework. These built-ins operate at a different operating model than running additional background servers. This typically means that everyone in the org needs to be an Elixir/Erlang expert instead of having experts in Redis, cloud native, ... For better or worse, this is likely a large obstacle for many orgs.

The argument for using Elixir/Erlang is also more difficult when you have large companies like Github and Shopify demonstrating that Ruby can scale.

EDIT:

Let's not forget that it's mostly the DB that slows CRUD apps down. Not the language or the framework.