Why isn't there a motherboard that I can use a SSD as a RAM

No.

RAM is volatile memory -- it requires constant power to keep data, and must be refreshed constantly as well.

Flash memory is non-volatile memory -- once data is written to it you can remove power and the data will remain saved.

Even though SSDs are far faster than hard drives, they are still the slowest part of modern computers (other than the user, of course). RAM is far faster than SSDs are.

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.

^+1 to CompulsiveBuilder

Using Flash as primary memory would dramatically slow down a computer and the volume of writes would wear out the cells in a matter of days, if not hours.

It's like asking why we don't use sports cars to haul freight. Different technologies for different purposes.