What's going on in Matthew 26:64? Why doesn't Jesus directly admit he's the Son of God when he is asked by the high priest?

You are right. In Matthew 26:64, Jesus (in his "trial") has been asked directly whether he is the Son of God, the Messiah, yet his response sounds, to non-Semitic ears, almost evasive: "You said [so]." Modern versions render the phrase as an affirmation. Is this honest?

Yes, it is honest, whether or not our ears are attuned to this ancient form of expression. In Matthew 26:25, the same construction appears:

Then Judas, who was betraying him, answered and said, “Rabbi, is it I?” He said to him, “You have said it.” (NKJV)

And again in Matthew 27:11:

Now Jesus stood before the governor. And the governor asked Him, saying, “Are You the King of the Jews?” Jesus said to him, “It is as you say.”

One further proof that this view is correct comes from the parallel passage in Mark 14:61b-64:

Again the high priest asked Him, saying to him, “Are you the Christ, the Son of the Blessed?” Jesus said, “I am..." Instead of the Aramaic expression "you said it," Mark's Greek has "I am." Thus there is no doubt that Jesus claimed to be the Christ.

Clearly you have said [it/so] in Greek renders an underlying Semitic affirmation. Or, in layman's language, a "yes" answer. Nothing indirect going on here.