Paradise and Hell Fire are as places of the unseen; and in the Perfect Wisdom of Allah, He decided to keep them unseen as a test for the mankind.
If Allah had laid open Paradise and Hell Fire for all to see, the test would become invalid for everyone who witnessed these two promised phenomena of Allah with their own eyes, they would immediately believe, and the test would not have any meaning!
WHAT IS THE LOGICAL NEED FOR THE HEREAFTER?
A good righteous person may live a good and pious life, and build many orphanages, and schools, and hospitals, and serve the poor and needy all his life. What reward can man possibly give such a righteous person? And what about all the people who benefited after his death, from the good works he laid the foundation for in his lifetime? Does he get absolutely no reward for those? Logic dictates that there must be a Day when the whole world should come to an end, and a Day of Accounting be held where the righteous get the reward for their good deeds. Otherwise it would not be a just system!
Conversely, what about an evil person who kills thousands of innocent people? What possible retribution can man take from this evil person? The maximum a court can do is decree to put him to death! The punishment for killing one person is death, and the killing for a thousand people is still only death in our limited world! Logic dictates that there must come a Day when the whole world should come to an end, and a Day of Accounting be held where full retribution is taken from the evil person.
If one leaves aside his set beliefs and prejudices, his own logic and reasoning will force him to accept that there has to be a Day of Judgment, where the righteous are rewarded for the good they did; and the evil is punished for the evil he committed!
To believe in the unseen is one of the conditions of faith (Imaan); and in Allah’s Perfect Wisdom and Knowledge, He decided to keep the rewards of Paradise and the punishment of the Hell Fire in the unseen.