I believe giants were supposed to trade with the player(came up with this after reading a note nailed to a sign post at Steamcrag camp available here http://www.uesp.net/wiki/Skyrim:Official_warning , also this would explain why they have a similar inventory to hunters) but at some point of the development the crew decided to keep them as they are now.
I also read a thread on this forum, where this modder noticed that Bethesda had a lot more planned with the Civil War quest-line, including the
recruitment of Giants to fight for one of the sides. Apparently, Bethesda had originally alot more planned for the Giants and the PC's interactions
with them. I bet you also remember that in the gameplay video launched by Bethesda prior to the game's release, there was actually an option to
talk with a giant walking right by. A real shame it isn't so, but we can always hope that Bethesda will try and complete and add the content they didn't
had time for in the first place. Giants are great!