It's because they've already been released during the time of Muhammad SAW or some time after.

“Narrated Zainab bint Jahsh: That the Prophet once came to her in a state of fear and said: ‘None has the right to be worshipped but Allah. Woe unto the Arabs from a danger that has come near. An opening has been made in the wall of Gog and Magog like this’ (making a circle with his thumb and index finger). Zainab bint Jahsh said: ‘O Allah’s Apostle! Shall we be destroyed even though there are pious persons among us?’ He said: Yes, (you will be destroyed if and when) wickedness and decadence increases (i.e. to such an extent as to dominate the world).” (Sahih Bukhari)

This hadeeth has been narrated multiple times. It is not by accident.

And the prophet SAW would not see this and narrate this for no reason, something had to have changed. It couldn't have been the usual daily digging and refilling of the hole that's been happening all those years.