Question
Why are angels NOT visible to human,
Answer
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
The angels are made by Allah The Almighty invisible to mankind for great wisdoms, one of which, as mentioned by the scholars, is that humans do not have the ability to see angels because they are created from light which no man is able to look at. In his commentary on the following Quranic verse (which means): {And if We had made him an angel, We would have made him [appear as] a man, and We would have covered them with that in which they cover themselves}[Quran 6:9], Ibn Katheer said: “It is reported from Ibn ‘Abbaas, may Allah be pleased with him, that he said, commenting on Allah's Saying (which means): {And if We had made him an angel, We would have made him [appear as] a man}: “Were an angel to be sent to them, he would have come to them only in the form of a man, because they could not look at the angels, because of the light (from which they are created).”
Another wisdom may be that Allah tests His slaves by asking them to believe in the angels who are, for them, a part of the unseen.
There may be further wisdoms we do not know of. Truly, it is Allah Who is the Wise, the Well-Acquainted.
Allah Knows best.
Fatwa answered by: The Fatwa Center at Islamweb