I think it might be worth pointing out the difference between religion and faith here, and between Christians as members of a church and as followers of Christ.
However much we may regret it, the fact is there are a variety of Christian churches and, when you analyse it, most of the differences between them are effectively man-made; we may all have the same Bible (not quite true, but that's another matter), but people interpret it differently and also take account of historical settings and changes in society.
It is one thing to be a church member, or even a strict Bible follower, quite another to follow Christ. As has been said the New Covenant which Jesus enters into with the world, while not negating the law, does supersede it; it is also true that the New Testament emphasis is not on the Law, but on love; not on observance but on morality; not on fear of God but on respect for and duty to Him; not on belonging to, or being chosen by God, but of our choice to follow Jesus.
To try and answer your question it is, at one level, simply because Christians are also human, but, at another level the question is only possible because we sometimes mistakenly assume "Christian" means a church member or a person who subscribes to a particular set of beliefs whereas it really means a follower of and believer in Jesus Christ and should be understood in terms of personal spirituality and devotion etc, not merely adherence to a particular text.