While it seems those symbols were formalized and set as standard by the International Electronics Committee in 1973 and was in common use before. While the early period where ON/OFF was clearly marked on equipment, there doesn't seem to be much documentation on the use of 1/0 or |/O - some people assume it's using the Boolean 1/0 for indicating power, others | for showing an open circuit connection. As UI developed, the actual physical interface became a singular button instead of a switch, and so graphic design probably made the jump from placing both on the image to putting both together on the button to superimposing them on each other.

Since it seems to have been dominant enough the IEC simply formalized it and the symbolic meaning, it probably was adopted and emerged in numerous places at once.

sources:

THE POWER CONTROL USER INTERFACE STANDARD. International Electronic Committee, 1973, www.energy.ca.gov/reports/2003-10-31_500-03-012F_APP.PDF.