I suspect that there are many practical reasons why you might not have a guard. For example they may get caught, or stuck in clothing or against straps.

However I suspect it is to do with the undertones of Japanese culture that can be seen within the Star Wars franchise films.<sup>1</sup> There are plenty of examples of this dotted about online, but it's largely understood that it was influenced if not subtly by the films of Akira Kurosawa.<sup>2</sup> 

With this in mind it's possible that the lack of guards could have been influenced by a cultural or artistic intention; Uesugi Kenshin<sup>3</sup> believed that a true warrior doesn't need a tsuba (guard) so their katana didn't have them. Pretty unique for daitō (long swords) in Japanese history.

This is what I've always understood to be the reason, but I would imagine that there are much more detailed breakdown of why this could be the reason elsewhere.

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<sup>1</sup> Japanese culture in star wards: [Link 1][1], [Link 2][2], [Link 3][3]

<sup>2</sup> Wiki reference to [Uesugi Kenshin][4]

<sup>3</sup> Wiki reference to [Akira Kurosawa][5]


  [1]: https://www.insidejapantours.com/blog/2015/12/17/sith-and-samurai/
  [2]: https://blog.gaijinpot.com/japan-and-the-jedi-how-japan-influenced-star-wars/
  [3]: http://www.openculture.com/2014/05/how-star-wars-borrowed-from-akira-kurosawas-great-samurai-films.html
  [4]: https://en.wikipedia.org/wiki/Uesugi_Kenshin
  [5]: https://en.wikipedia.org/wiki/Akira_Kurosawa