In 1976, the Z80 processor ran at 2.5 MHz and a typical DRAM access time was 500 ns. Now, processors run at 4 GHz and DRAM access time is 50 ns. Thus, processors are over three orders of magnitude faster but DRAM accesses are only one order of magnitude faster.
What are the technical reasons for this?
Why doesn't scaling help DRAM the same way it helps CPUs?
My hypothesis is that manufacturers increase DRAM capacity as transistors shrink, so the bit lines remain about the same length. Thus, the bit line capacitance doesn't scale down much and R-C delays on the bit lines keep DRAMs slow. Would a 1970s-sized 1-kilobit DRAM keep up with CPU speeds with modern technology, or are there other limiting factors?