Creating a route with a form and some content
As doc states


  defaults (required): Defines the default properties of a route.
  Provide one of the following to specify how the output is generated:
  
  _controller
  _form
  _entity_view
  _entity_list
  _entity_form
  _route


you can't list a _form and a _controller in the same routing.yml entry, only one of those keys is allowed. Either append that content via form API to your form, or render the form manually in your controller.