Since the question is vague and general, without specifying which dimension of the concept of imamate the occultation of Imam Mahdi (aj) contradicts, we will have to list some of the imam's major responsibilities and see if there actually is any contradiction or not.

Imamate, which is the continuation of prophethood, only differs with it in there being no receiving of revelation in imamate.  Therefore, the imam shares all of the responsibilities the prophet has, namely:

1- Taking measures in establishing an Islamic government and freeing the people from the oppression and illegitimate rule of tyrants.  The leader of a nation can't be expected to rise and try to take over no matter what the political circumstances of his time are.  It defeats the whole purpose.  That is why we see the prophet of Islam (pbuh) doing the same before his migration to Medinah; he would try to conceal things because of the circumstances he was living in.  This in no way flaws his prophethood; it only signifies the hard circumstances of that time.  The same goes with our time which is the time of the occultation of our imam; complicated and extenuating circumstances don’t allow him to rise.  So questioning why he doesn’t isn’t very logical.  On the other hand, he is mostly known for his rise and universal rule, so his occultation is in no way in conflict with the first responsibility and signifies that the people and Muslims still aren’t ready for his emergence.

2- Explaining Islamic teachings and guiding the people.  This is the second responsibility of the imam which is sometimes done by him directly and sometimes indirectly, just as the prophet of Islam (pbuh) would send others for the guidance of the people who were qualified for the  job.  This is also what the twelfth imam (aj) did and has done during the minor and major occultation; he assigned and has assigned “specific” and “general” representatives for the people to refer to regarding different issues.  These were some of the responsibilities of the imam in which his occultation has no conflict with.  There are also other benefits that the imam’s existence entails that have nothing to do with his responsibility and it is only because of the grace and love of Allah (swt) that we make use of these benefits, namely:

The imam is the “heart” and “soul” of this world; there are hadith that say if there was to be no imam in the world, the earth would swallow its inhabitants.

The imam is the intermediate link and mediator between Allah’s (swt) grace and mercy and us; if anything wants to reach us from Allah (swt), it is through and because of him.  His prayers are all accepted and answered and all of his intentions are good.  This matter has been pointed to in the ziyarahs of our imams.

We the children of Adam (pbuh) have sworn enemies who are led by Iblis (Shaytan).  But at the same time, the imam indirectly saves many of us from falling into their traps and guides us through the spiritual effects he has on us (that we might not be aware of).  These are only a few of the many benefits of an “absent” imam, who as the prophet (pbuh) said, is like the sun behind the clouds; it isn't seen, yet it benefits all.