"Why" really is the question here.
A product typically exists if it serves a purpose. Any riser cable for a DIMM would need to use the DIMM slot to begin with... If the DIMM slot was limited in such a way (cooler clearance, perhaps?), then there are low-profile options available to work around that.
I believe there was a line of Macs still used a daughterboard for memory in the last decade or so, but that's not a "riser" that would match your description.