Why is the Tracker not part of the SendinBlue's main API?

For the same reason as the Google Analytics Script is not part of the Google Analytics API. The Tracker can be used on client side, which means the Tracker ID will also be stored on client side. Unlike for the API key this is not an issue at it doesn't allow anyone to retrieve sensitive data from you account.

For now we don't provide any Marketing Automation API (to edit your workflows for example), but the day we do it will for sure be part of the main API.