For the record: As someone else may have posted by now (but I haven’t seen it yet), the reason is quite simple.
As E. Pluribus Unum pointed out, albeit rather tangentially, you surely wouldn’t want to eat something that would be “good” for decades. Similarly for alarm rider’s point about MRE’s.
The most likely reason your canned goods aren’t dated past 12/12/2012 is probably because for most canned goods, 4 years is their usual shelf life. That latest “batch” was probably produced around Dec 12th of 2008, thus the 4 year span.
I’d try buying some more canned goods again around April or May; I’d be willing to bet they would have an expiration date around 5/1/2013 for similar reasons.