What evidence is there that they have ever been able to physically materialize?
[Gen 6:1 KJV]
And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
[Gen 6:2 KJV]
That the sons of God saw the daughters of men that they [were] fair; and they took them wives of all which they chose.
[Gen 6:3 KJV]
And the LORD said, My spirit shall not always strive with man, for that he also [is] flesh: yet his days shall be an hundred and twenty years.
[Gen 6:4 KJV]
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare [children] to them, the same [became] mighty men which [were] of old, men of renown.
[Gen 6:5 KJV]
And GOD saw that the wickedness of man [was] great in the earth, and [that] every imagination of the thoughts of his heart [was] only evil continually.
[Gen 6:6 KJV]
And it repented the LORD that he had made man on the earth, and it grieved him at his heart.