Why did He say that they are foolish?
Radak (22): From the sea, you can recognize that I oversee lower beings, and do to them like My will (refer to 5:22:3:2). How did it cross your minds that I cannot do to you like My will if you transgress My Mitzvos?! Therefore, it is proper to call you a foolish nation without a heart for not understanding this. Or, if you understood it, and you have a heart to understand, your heart is rebellious. You recognize and deny.
Why does it say that they lack heart, sight and hearing?
Malbim: Man understands in three ways 1 . (a) Through intellect - but they (Yisrael) are foolish, without heart. (b) Through senses - but they do not see. (c) Through tradition - but they do not hear! The Navi will rebuke them (a) based on intellect - do you not fear Me?! (b) Based on senses - do you not tremble due to Me?!
Malbim: We find so - "Pen Yir'eh v'Einav uv'Aznav Yishma u'Levavo Yavin" (Yeshayah 6:10), "v'Lo Nasan Hashem Lachem Lev Lada'as v'Einayim Lir'os v'Aznayim Lishmo'a", (Devarim 29:3), "ha'Lo Sed'u ha'Lo Sishme'u ha'Lo Hugad
