On DOS it wasn't a thing because it didn't have a standard temp directory to begin with, so when the need arose, it was bolted on in an adhoc way (I remember the apps couldn't even agree on whether it should be %TEMP% or %TMP% or something else). Windows introduced it as a proper first party concept, but then moved it around so much that you pretty much had to use the API to retrieve the actual value if you wanted it to work. And then NT made that API return a path to a per-user directory by default.
On DOS it wasn't a thing because it didn't have a standard temp directory to begin with, so when the need arose, it was bolted on in an adhoc way (I remember the apps couldn't even agree on whether it should be %TEMP% or %TMP% or something else). Windows introduced it as a proper first party concept, but then moved it around so much that you pretty much had to use the API to retrieve the actual value if you wanted it to work. And then NT made that API return a path to a per-user directory by default.