Mainly because of catch-22catch-22. If your DRAM (as said already, RAM is very broad term. What you are talking about is called DRAM, with D for Dynamic) suddenly become non-volatile, people will call it NVRAM which is very different type of storage.
There is also a practical reason, currently no NVRAM (I mean true EEPROM-based NVRAM, with no power source required) types exists which allows an unlimited number of writes without hardware degradation.
Regarding DRAM-based mass storage devices: take a look at Gigabyte i-RAM (note the rechargeable Li-Ion battery, which makes it non-volatile for a while)
(source: techreport.com)