In short
The words have you not read
stress that the Pharisees should have known what he is explaining from Scripture. The scribes had read the passage, but not well; they did not apply it.
Jesus uses the phrase have you not read
in Matthew 12:3 and elsewhere in order to highlight that the Pharisees should have already been familiar with the principle he is explaining from Scripture. One may observe that Jesus knew very well that the scribes had, of course, read the relevant Scriptural passage. Yet they had not read it well; they did not apply it.
3 He said to them, “Have you not read what David did when he was hungry, and those who were with him: