Because 5GB of GDDR5 VRAM would probably run you about $400 add onto that the heat it would produce and its not that it isnt viable, its that its not physically possible. No PCI-E card can draw more than 400 watts of power, and that much power would roast the other components inside a system. They also dont know what new games will need in the future, the biggest performance increases come from changes to directx, you cant make a DX13 compliant card if no one has even formed a committee to discuss DX13 yet.
Why pay $800 today for a card that will play games in 4 years well when you can spend $100 every year and have a better card in the end and will have only spent $400, companies provide viable solutions to what is needed now, not what might be needed years in the future, even their crystal balls dont know the answer to that.