Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.
Note 18 at Mt 24:44: The Lord kindly but seriously pointed out the urgent need to be ready for His return. In the same way that a thief comes when people are least prepared, so the Lord will return in a time when people are not looking for Him.
This is not because our Lord desires to surprise all of us and see how many He can catch off-guard. On the contrary, this very prophecy is urging us to be watchful so we will be prepared. Jesus gave these signs of the end times to help us to be prepared. He was simply prophesying that there will be a condition of apathy in the latter days (Mt 24:12) that will tend to lull even the faithful to sleep if we don't take heed to His words.
In Noah's day, the people refused to prepare themselves for the coming flood because they were caught up with the affairs of this life (Mr 4:19) and didn't realize that judgment had come upon them until they were taken away in it (Mt 24:37-39). The Lord was prophesying the conditions that will be prevalent on the earth in general so that people in particular may take heed and spare themselves.