I created a meting in hotmail and sent it to myself.

When I get the email I see.

on the right of the screen I see these buttons

Clicking the down arrow allows a tentative acceptance clicking more allows you to select other calendars you want t accept the invitation to.

How Outlook works I have no idea. I have never used it's calendar and have not used Outlook at all in years..

You did not get the point!

Point is that all non-answered invitation are shown directly on calendar, so theoretically Lightening should parse emails and add invitation to Calender.

For more info how is called in Out look : http://theitbros.com/automatically-accept-meeting-requests-in-microsoft-outlook/

Mostly come the a problem at people with large amount of emails that are not processing it all the time and they miss invitation therefor.

I suggest you file a bug https://bugzilla.mozilla.org/ or even better hire a developer to implement your change. Thunderbird is not aimed at the corporate market and has no paid developers, so perhaps those busy folk need to pay for the feature if they want in anytime soon.