I don't know why the people on Earth wouldn't be able to see the angels at the 2nd advent of Jesus because Daniel saw Michael the angel according to Daniel 10:13, "But the prince of the kingdom of Persia withstood me one and twenty days: but, lo, Michael, one of the chief princes, came to help me; and I remained there with the kings of Persia."  Daniel knew Michael's name so he must have seen him. Daniel also saw Gabriel according to Daniel 8:15-19 & Daniel 9:15-27."

Zacharias, father of John the baptist, also saw Gabriel according to Luke 1:11-19 and talked with Gabriel. Likewise Mary, the mother of Jesus also saw Gabriel according to Luke 1:26-38, and conversed with him. Lot saw 2 angels in the city of Sodom according to Genesis 19:1, and Jacob went his way and the angels of God met him according to Gen. 32:1.  Mary Magdalene saw 2 angels within the sepulchre where the body of Jesus had lain according to John 20:11-13.  These are just a few examples.