To add to what Leaps said, the different in speed is dramatic. Magnetic disks work at speeds measured in milliseconds. SSDs work at speeds measured in microseconds. DRAM operates at speeds measured in nanoseconds. There are a thousand nanoseconds to a microsecond, and a thousand microseconds in a millisecond.
Also, SSD flash memory can only be written to a limited number of times, and MLC flash can only be erased in large blocks (and must be erased before it can be written). None of these features are suitable for the kind of memory you need for main RAM. Main RAM can be read and written a vast number of times, and you can read or write a single byte at a time (although the design of modern memory systems doesn't work exactly like that).
Flash memory and DRAM use quite different technology.