iTunes Sync Calendars does not show Outlook
A partial answer to my own question in an issue opened in Apple Communites.

Basically it says that, when using iCloud to sync your contents across all your devices (which is my case), it is not possible to use iTunes to sync the Calendars and Contacts.

From https://support.apple.com/en-gb/HT201253 : 


  If you use iCloud or other services like Apple Music to keep your content up to date across all of your devices, syncing through iTunes might be turned off. 


So instead of using iTunes, I just use iCloud for Windows which synced Calendars, Contacts &amp; Mail.
Below is an image of my work Calendar created in Outlook plus the imported ones from iCloud: