Because the parable of the fig tree means that summer is near. You're saying that summer has arrived. You're in effect saying,
Summer is the time for harvesting! According to Christ the harvesting is now, during this age of Gospel grace, then at the end of this age, Christ will send His angels to gather that which the laborlores (disciples) have harvested together through the message of the Gospel.
Matthew 9:37-38 (KJV) Then saith he unto his disciples, The harvest truly is plenteous, but the labourers are few; Pray ye therefore the Lord of the harvest, that he will send forth labourers into his harvest.
John 4:35 (KJV) Say not ye, There are yet four months, and then cometh harvest? behold, I say unto you, Lift up your eyes, and look on the fields; for they are white already to harvest.