I recently started learning about Composer, and being a WordPress enthusiast wondered about using Composer to build up a site with the WP core, plugins, and themes. There's some discussion about adding Composer support to core (https://core.trac.wordpress.org/ticket/23912), but that won't be resolved too soon from the looks of it.

There's nothing preventing developers from doing this on their own. In fact, there has been at least one proof of concept for an external package to do the work - see https://github.com/johnpbloch/wordpress-core-installer for source, and the package on Packagist - https://packagist.org/packages/johnpbloch/wordpress. Something I'm going to be trying out :)