Why is $form array empty when extending FormBase or NodeForm

Question

Why is $form not upcast with the requested bundle's fields for the default Form Mode for GET requests; and with the entities' values for other method requests?

My form:

namespace Drupal\tam_api\Form;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\Request;

use Drupal\node\NodeForm;
// use Drupal\Core\Form\FormBase; i've tried all of these as well...
// use Drupal\Core\Entity\ContentEntityForm;
// use Drupal\Core\Entity\EntityForm;
// Drupal\node\Form\NodeDeleteForm


class TamForm extends NodeForm {

  public function buildForm(array $form, FormStateInterface $form_state, $extra = NULL) {
    $form = parent::buildForm($form, $form_state);
    $res = array('fields'=>json_encode($form, TRUE));
    return new JsonResponse($res);
  }

  public function getFormId() {
    return 'tam_dynamic_form';
  }

  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm();
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);
    $module_data = system_rebuild_module_data();
  }

}

My route:

namespace Drupal\tam_api\Routing;
use Symfony\Component\Routing\Route;
use Symfony\Component\Routing\RouteCollection;


class TamRoutes {

  protected $formMap = array(
    'node' =>array(
        'tracks'=> array(
         'default'=>'/node/tracks',
         'add'=>'/node/tracks',
         'delete'=>'/node/tracks/{node}',
         'edit'=>'/node/tracks/{node}',
       )
     ) // and other entity types... 
  );

  public function routes() {
    $route_collection = new RouteCollection();

    foreach($this->formMap as $entity_type=>$bundles) {
      foreach($bundles as $bundle=>$routes) {
        foreach($routes as $verb=>$path) {
          $route = new Route('/forms'.$path);

          $perms = array('_access'=>'TRUE'); // TEMP while testing

          $opts = array(
            '_format' => ['json'],
            '_auth' => ['oauth2']
          );

          if ($verb != 'add') {
            $opts['parameters'] = array(
              $entity_type => array('type'=>'entity:'.$bundle)
            );
          }

          $defaults = ['_form' => '\Drupal\tam_api\Form\TamForm'];
          if ($entity_type == 'node') {
            if ($bundle === 'tracks') {
              $defaults = ['_entity_form' => 'node.tracks'];
            }
          } else if ($entity_type == 'group_content') {
            $perms['_group_member'] = 'TRUE';            
            if ($verb != 'add') {
              $opts['parameters']['group_content'] = array('type'=>'entity:group_content');
            }
          }

          $route->setDefaults($defaults);          
          $route->setRequirements($perms);
          $route->setOptions($opts);

          if ($verb === 'add') {
            $route->setMethods(['POST', 'GET']);
          } else if ($verb === 'edit') {
            $route->setMethods(['PATCH', 'GET']);
          } else if ($verb === 'delete')  {
            $route->setMethods(['DELETE', 'GET']);
          } else {
            $route->setMethods(['GET']);
          }

          $route_collection->add($entity_type.'.'.$bundle.'.'.$verb, $route);
        }
      }
    }
    return $route_collection;
  }

}

My module:

function tam_api_entity_type_build(array &$entity_types) {
  tam_api_module_entity_type_alter($entity_types); // _alter isn't seeming to fire by itself?
}
function tam_api_module_entity_type_alter(array &$entity_types) {
  $form_modes = \Drupal::service('entity_display.repository')->getAllFormModes();
  foreach ($form_modes as $entity_type => $display_modes) {
    if ($entity_type !== 'node') {
      continue;
    }
    $node_type = $entity_types[$entity_type];
    foreach ($display_modes as $machine_name => $form_display) {
      $default_handler_class = '\Drupal\tam_api\Form\TamForm';
      $node_type->setFormClass($machine_name, $default_handler_class);
    }
  }
}

ERRORS are as follows:

GET: /forms/node/playlists/31694?_format=json: (which routes through _entity_form) Error</em>: Call to a member function getEntityTypeId() on null in Drupal\Core\Entity\EntityForm-&gt;getBaseFormId() (line 79 of core/lib/Drupal/Core/Entity/EntityForm.php.

GET: /forms/node/tracks/32434?_format=json: (which routes through _form) Argument 1 passed to Drupal\Core\Entity\EntityForm::setEntity() must implement interface Drupal\Core\Entity\EntityInterface, string given, called in /Users/user/Developer/trackauthority/tam-d8/web/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php on line 74 in Drupal\Core\Entity\EntityForm->setEntity()

GET /forms/node/tracks?_format=json (which routes through _form Missing bundle for entity type node in Drupal\Core\Entity\ContentEntityStorageBase-&gt;doCreate()</em> (line 86 of core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php). >Drupal\Core\Entity\EntityStorageBase-&gt;create(Array) (Line: 381)

Answer 1

When you return a response instead of a Form API array

  public function buildForm(array $form, FormStateInterface $form_state, $extra = NULL) {
      ...
    return new JsonResponse($res);
  }

the form is reduced to a request/response pattern of a controller. It is no longer possible to use it as a Drupal form.