Keynesian economists believe that real gdp might be less than full employment because

Keynesian economists believe that the real GDP might be less than full employment because the consumer demand in the economy is lesser than required.

The Keynesians support the concept of massive spending by the government, as well as massive debt and monetary inflation. They also believe that the concept of saving was a blow on demand because expenditure tends to be less during times of recession. According to them, this only aggravated the recession.