Probably didn't work out for the reasons you mention.
Next question: What prevents Intel or AMD combining logic process and DRAM process or Samsung and others combining logic in their DRAM ships?
Integrating CMOS and DRAM might be impossible to do so that the price/speed is less than using separate chips. Combining two processes increases the price. CPU/GPU makers don't have the latest DRAM knowledge. Reverse is also true. Partnership is required.
Then there are technological problems: There are yield differences in different processes. CPU/GPU's operate in high temperatures. DRAM's technology needs to adjust or there needs to be halfway solution.
It's possible that in some time in the future new technology called STT-MRAM could replace replace low-density DRAM and SRAM and it could be integrated into logic because it can use existing CMOS manufacturing techniques and processes. It will take time. (STT = Spin-Transfer Torque)