There appears to be no such straight forward solution from Microsoft. There are third party software/plugins that do that and they are not free, not to mention some are complicated to use.
The idea with Mail Merge is not just mass mailing, but it has features like Greet Lines where you can use variables for specific recipient's addresses/names, making the letter look more personal...Dear Tim, etc.
However, I did find the best solution (thus far) to do this (source):
In Outlook, just go to Send/Receive>Work Offline.
This way, Mail Merge in Word will safe all Mail Merge emails to the Outbox folder in Outlook, where one can edit/attach the mails one by one. Yes, one by one, but at least we don't have to do one by one email from scratch, the main content is in every mail.
After that, set Work Offline to online and Outlook will send them all at once (or click the Send All button on top under Send/Receive menu).