1) QB’s don’t have 180 degree field vision. They can’t see everything at one time. Not Brady. Not Kelly. Not Mahomes. Not anyone.
2) Allen was not at that point in his progression. He saw Diggs, his first read, have an open look. But by the time Diggs uncovered, three defenders were about to demolish Allen.
3) For both of these reasons, and many more, Allen did not look to Kincaid.
You can find “open” receivers for every QB. But the context matters.