It would be prohibitively expensive. Several megabytes of cache already take up about half of the CPU die size. http://techreport.com/review/21987/intel-core-i7-3960x-proce...

Think about how much larger a CPU would have to be to include an entire gigabyte of cache, much less multiple gigabytes.

First, SRAM caches are still faster than DRAM.

Second, DRAM is made differently from CPUs, and it is difficult to use two different processes on the same silicon.

Third, squeezing 4GB of DRAM onto a single CPU die? How?