But it's not.
"...the Eemian was punctuated by many short-lived cold events, as shown by variations in electrical conductivity (a proxy for windblown dust, with more dust indicating colder, more arid conditions) and stable oxygen isotopes (a proxy for air temperature) of the ice were used by these workers infer the climatic conditions during the Eemian. The cold events seemed to last a few thousand years, and the magnitude of cooling was
similar to the difference between glacial and interglacial conditions; a very dramatic contrast in climate. Furthermore,
the shifts between these warm and cold periods seemed to be extremely rapid, possibly occurring over a few decades or less...."
Sudden climate changes in the recent geological record
And then there were the D-O events of the last glacial period which also had 5C swings over several decades.