@nspace@infosec.exchange some systems don't need to have a 36- or 72-bit-wide memory, so the common memory chip widths don't have a factor of 9 built in.
(the extra bits above a power of 2 are used for error correcting codes to recover from memory bit flips).
also, there tends to be a number of bits you can fit on a chip inexpensively today, and you can use that as x16 if you don't need many words of memory (and thus less stuff on your board) or x4 if you do (and fewer chips on each data line).
all of the above comes from being vaguely near embedded hw design that uses ddr, but it's not actually my job.
