Oh, actually, I remember why this exists. It's for variable hook names, e.g. in `wp_transition_post_status()`:

`do_action("{$old_status}_to_{$new_status}", $post);`

These look like `$old_status . '_to_' . $new_status`, so for consistency with that, the static strings have the delimiters. I'm happy to change this behaviour as long as we get something clean.