Reusable blocks are referenced, and that is a different tool

A reusable block is a post of type wp_block and every use is a reference to it, which is a genuine component and has the consequences of one.

// what is stored in the post that uses it
<!-- wp:block {"ref":4102} /-->

// so a reusable block is:
//   editable in one place, changing every use
//   deletable, leaving every use showing nothing
//   not exportable with the content that references it

wp post list --post_type=wp_block --fields=ID,post_title

Deleting one silently empties every post that used it, which is the failure mode that makes people distrust the feature — there is no reference count anywhere in the interface. It is the right tool for a call to action repeated across forty pages and the wrong one for a layout each page should adapt. Migrating a site between environments needs the wp_block posts to travel, which the usual export includes and a partial one does not.