Because generally you don't merely right click. You right click and then have to navigate a small menu of options to find the one you want. The big button on the ribbon is a huge target and is generally easier for users to hit and click.

And for some reason, I personally find right-clicking with a track-pad much more difficult than right clicking with a mouse, although I haven't taken a lot of time to analyze why as I generally use a desktop machine with mouse.