Why do you never see marines wearing swords?

Swords are worn by Marines based upon their assigned positions in ceremonial situations. All officers are required to own their own "Mameluke" sword and while many SNCOs and NCOs own their own sword modeled on U.S. Army M1850 foot officers' sword introduced in 1859, with significant modifications in 1875, 1918. If an NCO or SNCO does not own his sword, units have them in stock as organizational equipment.
If you desire to see them in use, google "USMC Evening Parade" and watch any of the videos. Better yet, look at the evening parade schedule and attend the Parade in Washington DC. It is an inspiring and entertaining event, free admission.

At The Basic School, (A course all Marine 2ndLt's attend) I remember a video clip of a search and destroy mission in Vietnam. In the middle of it all is some Lt with a Sam Brown Belt over his sateens and a a mameluke sword drawn as he goes through the doorway of a hut. We all laughed and watched that short segment two more times.

In my day, most Marine officers swords are ceremonial and not really a weapon, they didn't even have a sharpened edge. The ceremonial swords were about $600 in the early 80's. My Dad sent to the English Sword maker Wilkinson, and bought me and edged Wilkinson that was weapon quality. I think he paid around $3000, plus shipping.

At the first sword manual class, I was told, "Nice sword, don't bring it to class again." So I have two swords.

Thank you everyone for the informative comments. Also thanks for what you all have done for us!