Include callback from another file in plugin on widgets_init hook

I am having trouble understanding why I cannot include a callback on the action hook ‘widgets_init’ in my plugin’s index.php from another file using include(‘path’)like I normally do .
Generally , to keep my index.php file of my plugin clean I include callbacks like below

include( 'includes/init.php' );
add_action( 'init', 'recipe_init' );

But for the ‘widgets_init’ hook I cannot directly include like below :

include( '/includes/widgets.php' );
add_action( 'widgets_init', 'r_widgets_init' );

Which thorws an error :

Warning: include(/includes/widgets.php): failed to open stream: No such file or 
directory in C:\xampp\htdocs\clean1\wp-content\plugins\recipe2\index.php on line 29
Warning: include(): Failed opening '/includes/widgets.php' for inclusion 
(include_path='C:\xampp\php\PEAR') in C:\xampp\htdocs\clean1\wp-content\plugins
\recipe2\index.php on line 2 :

Instead something like below works :

include( dirname(RECIPE_PLUGIN_URL) . '/includes/widgets.php' );
add_action( 'widgets_init', 'r_widgets_init' );

I checked the functions.php of my theme and there we seem to be able to include a
callback on the same hook directly :

include( get_theme_file_path( '/includes/widgets.php' ) );
add_action( 'widgets_init', 'ju_widgets' );

I am curious as to why it doesnt work in case of plugins while in the theme it
works ? I checked and found ‘widgets_init’ hook to be trigerred pretty late
after ‘plugins_loaded’ hook is fired so all my plugin code should be available by
the time ‘widgets_init’ is fired or am I misunderstanding how WP loads ?