Entity access controler not called from custom route
I guess you need to do a route parameter upcasting:

entity.politician.about_me:
  path: '/politician/{politician}/about-me'
  defaults:
    _controller: '\Drupal\pw_basic\Controller\PoliticianFrontend\AboutMe::build'
    _title_callback: '\Drupal\pw_basic\Controller\PoliticianFrontend\AboutMe::title'
  requirements:
    _entity_access: 'politician.view'
  options:
    parameters:
      politician:
        type: entity:politician