In Drupal 8 module creation, why don't I have to put the directories "modules" and "src" in the namespace of my yaml file?
"Why", in a general sense, is for convenience. Drupal 8 uses PSR-4 autoloading and maps the respective module namespaces to each module's src/ directory. See drupal_classloader_register():

function drupal_classloader_register($name, $path) {
  $loader = \Drupal::service('class_loader');
  $loader-&gt;addPsr4('Drupal\\' . $name . '\\', \Drupal::root() . '/' . $path . '/src');
}