How can I add a link to the main menu from a custom module?
You are missing the key menu_name that will permit the addition of your new menu item into your main menu.

my_module.list:
  title: 'List All Computers'
  description: 'Show all known computers'
  route_name: my_module.listAllComputers
  parent: main
  menu_name: main


The parent key is not mandatory, so if you want your link to be shown on the first level of your main_menu, you don't need it.