All of Torah is wrapped up in these leather straps and boxes with their finely written parchment scrolls.
Because this is the essence of every mitzvah:
To bind your heart, your mind and your action together into a single wholeness, wrapped up inextricably with the One who spoke and the world came into being.
It'd be nice if those who are left-handed had equal opportunity, but they don't because of the Tefillin Shel Rosh - both the scrolls inside and the Shins on the outside.
No one seems to even be willing to make such a case - and yet, none will cite the actual, specific source of the halachah that they rely on to not make such a case.