Because you need to overclock your memory controller to obtain those speeds... Simple as that...
The memory controller in AMD's CPU are really only guaranteed by AMD to run memory up to 1333.. It's a limitation of the CPU..
I believe Intel's CPUs do a bit better in this regard, but only when you look at their higher end chips..
Ya, you can run memory at 1600 on an AMD board, but it's pointless... The CPU is not going to be able to keep up with it.. So the CPU has a memory controller appropriate for the performance of the CPU.. Which basically means that for AMD, any memory faster than 1333 is not going to provide any benefit to be worthwhile... Better off buying memory with faster timings..
We're going to see some new AMD chips out next year for Socket AM3, and I believe they all start with memory controllers at 1600, though I'm not really sure.. 1600 is more than enough memory bandwidth for AMD's upcoming 8-core CPUs.. Running 1600 on existing quad-cores, is silly.. But if you want to get the absolute most out of next gen chips without replacing memory, then it's not a bad idea to start buying 1600 memory now, assuming you don't already have memory.
Even still, running 1333 Memory on an AMD 8-core CPU likely won't be much of a bottleneck, I believe the 12-core Opterons for servers/workstations are already running with similar bandwidth without having any memory bottleneck to speak of. With dual-channel, your bandwidth is multiplied by 2, so again, that's a LOT of memory bandwidth we're talking about here..
Remember, DDR3 is just now coming down in price so it's just now entering the average joe consumer market (ie: Walmart)... Any speeds above 1333 is overkill for the majority of CPUs (even overclocked) on the market. It will be a few years before DDR3 starts to show it's age.. And by then, DDR4 will be around the corner..
Edit: Allow me to point you to this table to show you how ridiculously fast some of the memory on the market is right now..
DDR3-1600 = 12,800 Megabytes (12.8GB) a second..
That's pretty darn fast..