Because he was asserting that there’s a 3-fold division in the OT: Prophets, the Law, and the Psalms.
The 7 books from the deuterocanon fit quite nicely into these 3 divisions.
Sorry, no they don’t. The Catholic church divides the 7 Apocrypha books in with the OT Hebrew Scriptures in a FOUR-fold division, not a THREE-fold division:
Torah: Genesis, Exodus, Leviticus, Numbers, Deuteronomy;
Historical books: Joshua, Judges, Ruth, 1 Samuel, 2 Samuel, 1 Kings, 2 Kings, 1 Chronicles, 2 Chronicles, Ezra, Nehemiah, Tobit, Judith, Esther, 1 Maccabees, 2 Maccabees;
Sapiential (Wisdom) books: Job, Psalms, Proverbs, Ecclesiastes, Song of Solomon, Wisdom, Sirach;
Prophetic books: Isaiah, Jeremiah, Lamentations, Baruch, Ezekiel, Daniel, Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi
The THREE-fold division of the Law, the Prophets, & the Psalms is what the Pharisees recognized (which is who Jesus addressed & even CA Jimmy Akin affirms), & what later Jews recognized in their TaNaKh that does not include the 7 Apocrypha books:
Torah (“Teaching” – Five Books of Moses)
• Genesis, Exodus, Leviticus, Numbers, Deuteronomy
Nevi’im (“The Prophets”)
• The ‘Major Prophets’
o The ‘Former Prophets’ – Joshua, Judges, (I & II) Samuel, (I & II) Kings
o The ‘Latter Prophets’ – Isaiah, Jeremiah, Ezekiel
• The ‘Minor Prophets’ (1 Book)
o Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi
Ketuvim (“The Writings”)
• The ‘Poetic Books’ – Psalms, Proverbs, Job
• The ‘Five Rolls’ (‘Megilloth’) – Song of Solomon, Ruth, Lamentations, Ecclesiastes, Esther
• The ‘Historical Books’ – Daniel, Ezra-Nehemiah, (I & II) Chronicles
Sorry, historically & Scripturally, by Jesus referring to “the Law, the Prophets, & the Psalms,” He wasn’t including the 7 Apocrypha books.