The answer to this question lies in explaining why the occultation took place, what the awaiting of the Mahdi (aj) truly means, and what the benefits and outcomes of having a concealed imam are.

It has been said in our hadiths that one of the reasons for the occultation is so that he won't have to pledge allegiance to anyone, or so that he will be protected from being killed.  Of course, these reasons can't be considered reasons for him necessarily being born a long time before his emergence, but there are other reasons which make such a matter for sure:

a) If the imam is to establish a new civilization unlike previous ones, it takes time for the people to realize that their own methods of life and civilizations aren’t up to the task of managing humanity and fulfilling its high goals and ideals, therefore people need to see such a matter for themselves in order to look forward to, be prepared and accept his universal government and see it as the solution to all their problems.

b) The occultation of the imam is a means of testing the servants of Allah (swt).  Why Allah's (swt) servants need to be tested is the same reason for why there has to be an unseen imam that is a means of testing the people.

c) Next to the concept of occultation, there is the concept of anticipation and awaiting; the two are inextricably intertwined.  Anticipation brings both personal spiritual development and social development, and although these two are reachable even without belief in a live and present (although unseen) imam, yet belief in a live and present imam that might emerge any minute, strengthens and speeds the two up.

In our hadiths, the imam during his occultation has been compared to the sun behind the clouds; it benefits all, yet isn’t seen.  Benefits of this sun can be:

1- Just like how the commander of an army gives hope to his soldiers when he is alive and in the battlefield, a live and present imam gives hope to his followers as well.

2- Knowing that the imam is alive and monitoring our actions has a deep influence on individuals, causing them to work harder on building themselves spiritually.

3- All infallible imams and divine apostles, Imam Mahdi (aj) being the last of them, have been the guardians of Islam in their time, bearing all the secrets, reasons and proofs of Islam, both those that are apparent and manifest, and those that are concealed and hidden.  It is on this basis that the protection and safeguard of religion and its concepts is their responsibility, and they are the ones who have to prevent Islam, which is the "seal of religions" as a result of the closing of the door of revelation to all after it, [hence, its superiority to other religions] from undergoing the slightest alteration and distortion, so it can be brought and applied to the everyday lives of normal people.

4- Some actually possess the capacity and are qualified to "climb the ladder" and ascend to the skies and make direct use of the sun instead of it being behind the clouds [meaning that they see or make direct contact with the imam, or might not see him, yet they are benefited in a special way that others aren’t].

5- We believe that the imam (aj) has authority over the hearts and actions of the people and in reality, guidance goes through him and it is his responsibility to help others reach their destinations, all of these are contingent upon the imam being live and present.

6- The aim of creation is the perfection of man, thus, if there is to be a day in which there is no perfect individual on the face of the planet, the world will cease to exist, therefore there is always a need for at least one perfect individual to be living on it, hence the need for a live imam.

7- In order for there to be a link  between the realm of essential unity and pure multiplicity, there is a need for a third manifestation that serves as a channel between the two worlds, that channel being the perfect and complete individual, who is none other than an infallible imam.