theme_imagecache() doesn't set height and width

in imagecache.module

function theme_imagecache($namespace, $path, $alt = '', $title = '', $attributes = null) {
  // check is_null so people can intentionally pass an empty array of attributes to override
  // the defaults completely... if
  if (is_null($attributes)) {
    $attributes['class'] = 'imagecache imagecache-'. $namespace;
  }
  $attributes = drupal_attributes($attributes);
  $imagecache_url = imagecache_create_url($namespace, $path);
  return '<img src="'. $imagecache_url .'" alt="'. check_plain($alt) .'" title="'. check_plain($title) .'" '. $attributes .' />';
}

You can see that height and width are not set, while it is done for example in theme_imagefield_image() in imagefield.module

function theme_imagefield_image($file, $alt = '', $title = '', $attributes = NULL, $getsize = TRUE) {

  $file = (array)$file;
  if (!$getsize || (is_file($file['filepath']) && (list($width, $height, $type, $image_attributes) = @getimagesize($file['filepath'])))) {
    $attributes = drupal_attributes($attributes);

    $path = $file['fid'] == 'upload' ? $file['preview'] : $file['filepath'];
    $alt = empty($alt) ? $file['alt'] : $alt;
    $title = empty($title) ? $file['title'] : $title;

    $url = $file['fid'] == 'upload' ? url($file['preview']) : file_create_url($path);
    return '<img src="'. check_url($url) .'" alt="'.
        check_plain($alt) .'" title="'. check_plain($title) .'" '. $image_attributes . $attributes .' />';
  }
}