Form API placeholder attribute rendered as "- Select -"

In a custom Drupal 8 module, I have a couple of Form API fields of '#type' => 'select':

<?
    $form['form-right']['field_role'] = [
      '#type' => 'select',
      '#title' => $this->t('Role'),
      '#title_display' => 'invisible',
      '#required' => TRUE,
      '#options' => [
        'foo' => $this->t('Role Foo'),
        'bar' => $this->t('Role Bar'),
        'baz' => $this->t('Role Baz'),
        'quux' => $this->t('Role Quux'),
        'plugh' => $this->t('Role Plugh'),
        'xyzzy' => $this->t('Role Xyzzy'),
      ],
      '#attributes' => [
        'placeholder' => $this->t('Role'),
      ],
    ];

    $form['form-right']['field_contact_reason'] = [
      '#type' => 'select',
      '#title' => $this->t('Contact Reason'),
      '#title_display' => 'invisible',
      '#required' => TRUE,
      '#options' => [
        'foo' => $this->t('Contact Reason Foo'),
        'bar' => $this->t('Contact Reason Bar'),
        'baz' => $this->t('Contact Reason Baz'),
        'other' => $this->t('Other'),
      ],
      '#attributes' => [
        'placeholder' => $this->t('Contact Reason'),
      ],
    ];

Before a value is selected, the dropdown select picklists display the string '- Select -':

The client would like them to display the value in the placeholder attribute instead.

Have I done something wrong?