Putting more memory bumps up the cost of the console. By your logic, they could put in 16 GB of GDDR5 and sell the console at $1000. I bet everyone would buy it then.
Do you realize you can buy GDDR5 modules in bulk for under $2 per 500mb module?, so you could bump a systems memory by 4gb for a BOM increase of at most, $16?
Do you think a BOM increase of $16 would increase the consoles price from the rumored $400 mark, to $1000?