Roughly speaking, CPU wants smaller space and RAM wants bigger space. At a high-level take:
* CPU design is the most expensive space due to it having the greatest quality and capability requirements. RAM is mostly just a very, very large repetitive structure, so more space better.
* A CPU fault can be potentially corrected by microcode changes to route around the damage (ie part binning). RAM cannot generally take faults.
* DRAM is simpler to make, "just" a capacitor, but capacitance leaks over time; which, means generating resistive heat in an area that we want as little heat as possible. You could use SRAM (two transistors) but now you have substantially more complex part to fail.
* DRAM quality requirements are much less stringent if you make just bigger cells.
