Adam Clarke on Matthew 23:2

The scribes and the Pharisees sit in Moses' seat - Εκαθισαν. - They sat there formerly by Divine appointment: they sit there now by Divine permission. What our Lord says here refers to their expounding the Scriptures, for it was the custom of the Jewish doctors to sit while they expounded the law and prophets, (Mat 5:1; Luk 4:20-22), and to stand up when they read them.

By the seat of Moses, we are to understand authority to teach the law. Moses was the great teacher of the Jewish people; and the scribes, etc., are here represented as his successors.