Because people keep complaining about missing them. If you find some please post them.

PC / Mac: WT5TB-XC5ZC-CX3T3-BBT3B-B35WB
Xbox 360: W3KTJ-FZ9CS-XS6BR-K6JJ3-XCFXT
PlayStation 3: WTWBJ-HKZ9C-TCCWB-ZT5JJ-6HRZS

THe above are worth 5 keys each for borderlands 2. they probably wont be working for too much longer, so use em while you can.