If you ask any haafiz, they will tell you that it's easy to memorize but harder to retain in memory. The revision is a life-long effort that remains with you till your last breath. Prophet ﷺ gave us a strong reason to keep revising.
Narrated Ibn Umar (may Allah be pleased with him):
Allah's Apostle ﷺ said, "The example of the person who knows the Qur'an by heart is like the owner of tied camels. If he keeps them tied, he will control them, but if he releases them, they will run away."[Sahih al-Bukhari 5031]
Narrated Abu Musa (may Allah be pleased with him):
The Prophet ﷺ said, 'The example of a believer who recites the Qur'an is that of a citron (a citrus fruit) which is good in taste and good in smell. And the believer who does not recite the Quran is like a date which has a good taste but no smell. And the example of an impious person who recites the Qur'an is that of Ar-Rihana (an aromatic plant) which smells good but is bitter in taste. And the example of an impious person who does not recite the Quran is that of a colocynth which is bitter in taste and has no smell."[Sahih al-Bukhari 7560]