why does blog page ignore template [closed]

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Refer to the Template Hierarchy Codex entry.

The template hierarchy for the blog posts index is as follows:

  • home.php
  • index.php

There is no case where the blog posts index will use a page template, either the default page template or a custom page template.

The determination is controlled by wp-includes\template-loader.php. Refer to Line 31:

elseif ( is_home()  && $template = get_home_template()   ) :

And get_home_template() is defined in wp_includes\template.php:

function get_home_template() {
    $templates = array( 'home.php', 'index.php' );

    return get_query_template( 'home', $templates );
}

So, as you can see, the blog posts index never looks for any variation of a page template.