Because there is no need. Lunar race was for dominating ideologically. The USSR decided to 'lose' in exchange for more favorauble grain prices. Would NASA really be able to fly to the Moon, why'd they care to abandon Saturn V and start to buy Kuznetsov engines from Russia (effectively 40 years old tech). They simply were not on the Moon. Saturn was a weak rocket, unable physically to deliver Apollo module to the Moon. Also look for details of the Southwind icebreaker mission in 1970. 8 september. Sorry dude, its all lies, man never been on the Moon. Soviets did not wanted, Americans simply could not.
If that was the case then why has the 60s 'faking' not been discovered? If a 1960's faking is undetectable in 2012 then I expect a 2012 faking would be even more convincing. Thus there must be some other reason for not faking a mars landing.
PS: The exact name for my line of argument is 'begging the question'. Yes, that 'begging the question'. The one grammar nazis internet wide have been referring to since 1994. Bet you thought you'd never see it.