Why is styling comments so complex?

If you prefer simple template files, you can do that with custom comment callbacks too.

Call wp_list_comments() with a custom callback handler:

wp_list_comments( 
    array( 
        'callback' => 'custom_comment_callback', 
        'style'    => 'ol' 
    ) 
);

Now make that callback function very simple:

function custom_comment_callback( $comment, $args, $depth )
{
    include 'comment-template.php';
}

And now you can use comment-template.php like any other template.

Here is a simple example showing you what variables are available:

<?php # -*- coding: utf-8 -*-
print '<pre>$comment=" . esc_html( var_export( $comment, TRUE ) ) . "</pre>';
print '<pre>$args="    . esc_html( var_export( $args, TRUE ) )    . "</pre>';
print '<pre>$depth="   . esc_html( var_export( $depth, TRUE ) )   . "</pre>';

You can also use the functions body of an existing callback handler like that from Twenty Twelve here, and it will work:

<?php # -*- coding: utf-8 -*-
$GLOBALS['comment'] = $comment;
switch ( $comment->comment_type ) :
case 'pingback' :
case 'trackback' :
    // Display trackbacks differently than normal comments.
    ?>
    <li <?php comment_class(); ?> id="comment-<?php comment_ID(); ?>">
        <p><?php _e( 'Pingback:', 'twentytwelve' ); ?> <?php comment_author_link(); ?> <?php edit_comment_link( __( '(Edit)', 'twentytwelve' ), '<span class="edit-link">', '</span>' ); ?></p>
    <?php
            break;
        default :
        // Proceed with normal comments.
        global $post;
    ?>
    <li <?php comment_class(); ?> id="li-comment-<?php comment_ID(); ?>">
        <article id="comment-<?php comment_ID(); ?>" class="comment">
            <header class="comment-meta comment-author vcard">
                <?php
                    echo get_avatar( $comment, 44 );
                    printf( '<cite class="fn">%1$s %2$s</cite>',
                        get_comment_author_link(),
                        // If current post author is also comment author, make it known visually.
                        ( $comment->user_id === $post->post_author ) ? '<span> ' . __( 'Post author', 'twentytwelve' ) . '</span>' : ''
                    );
                    printf( '<a href="https://wordpress.stackexchange.com/questions/97471/%1$s"><time datetime="%2$s">%3$s</time></a>',
                        esc_url( get_comment_link( $comment->comment_ID ) ),
                        get_comment_time( 'c' ),
                        /* translators: 1: date, 2: time */
                        sprintf( __( '%1$s at %2$s', 'twentytwelve' ), get_comment_date(), get_comment_time() )
                    );
                ?>
            </header><!-- .comment-meta -->

            <?php if ( '0' == $comment->comment_approved ) : ?>
                <p class="comment-awaiting-moderation"><?php _e( 'Your comment is awaiting moderation.', 'twentytwelve' ); ?></p>
            <?php endif; ?>

            <section class="comment-content comment">
                <?php comment_text(); ?>
                <?php edit_comment_link( __( 'Edit', 'twentytwelve' ), '<p class="edit-link">', '</p>' ); ?>
            </section><!-- .comment-content -->

            <div class="reply">
                <?php comment_reply_link( array_merge( $args, array( 'reply_text' => __( 'Reply', 'twentytwelve' ), 'after' => ' <span>&darr;</span>', 'depth' => $depth, 'max_depth' => $args['max_depth'] ) ) ); ?>
            </div><!-- .reply -->
        </article><!-- #comment-## -->
    <?php
        break;
    endswitch; // end comment_type check