Why does drupal use .yml and not officially recommended .yaml
Drupal uses '.yml' because Symfony uses '.yml'

This was discussed in https://www.drupal.org/project/drupal/issues/2091669

I am assuming that '.yaml' would not work, but I haven't tried it. For consistency reasons it is best to stick to '.yml'.

EDIT: Looks like Symfony has started recommending '.yaml' from version 4.0. Can be seen by comparing this page https://symfony.com/doc/4.0/routing.html with the 3.4 version