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Flash Memory is considerably slower than DRAM, and the way they are used by operating systems is very different.
A typical DRAM has a transfer rate of approximately 2-20GB/s, whereas typical SSDs have a transfer rate of 50MB-200MB/s. So it's one to two orders of magnitude slower.
Furthermore, the way it's used is very different. DRAM is far more flexible and is truly random access -- any word, any time. By comparison, flash writes have to erase an entire block at a time before writing to it; and there are other problems that need to be dealt with such as wear leveling and bad blocks.