It is because he does not come on a literal cloud. In Luke 18:8 Jesus says, "I tell you, he will vindicate them speedily. Nevertheless, when the Son of man comes, will he find faith on earth? Why did Jesus say this? There are many people around the world who are doing their utmost to prepare for his coming, although their faith may not be perfect. Who would fail to recognize the Lord coming on a literal cloud? Even non-believers would recognize him and come to him. Is it because someone will prevent them from doing so? Certainly not. Difficulties do not necessarily weaken a man's faith. Then would it be because he will come in exactly the same way as Jesus did the first time?
Two thousand years ago, when Jesus came, there was much faith among the people. Day and night they prayed in the temple and memorized their commandments. They tried hard to keep all the commandments and laws that God ordered them to keep. They faithfully offered their tithes and they fasted. In this sense, they had much faith in believing in God, yet they did not understand the true meaning of faith. They were being prepared to recognize the son of God who was to be sent to them. From this viewpoint, Jesus could not find any faith on earth! Similarly, today there are millions of good Christians waiting for the arrival of the Lord of the Second Advent, but if the Lord comes in the same way as he did before, will he find the faith that will enable Christians to recognize him?
To emphasize once again, from the lessons we have previously learned from the experience of God's working in history, and judging from the scriptural passages quoted before, the Second Advent will occur as at the First Coming, by the Messiah's birth in the flesh, born of a woman. Indeed, he comes as the Son of man.