3Knowing this first, that there shall come in the last days scoffers, walking after their own lusts,
4And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation.
5For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water:
6Whereby the world that then was, being overflowed with water, perished:
7But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.
Many still continue in their lives, unknowing of the presence of God. There could be a variety of reasons, but mostly from ignorance. If you will just take time to find out more, and learn what the bible has to say and dig into its lessons, you will find treasures within - the truth of the world. Unfortunately, most are content not knowing, or choose not to. Sometimes, ignorance presents itself to be a temporal bliss, for the bible contains hard truths that may not be readily accepted by some. Sometimes, unpleasant experiences give rise to obstacles that make us shun away from God. I know of at least a couple of people who shun christianity because it was their ex-partner's religion (I find their actions incredibly dumb btw). Sometimes, we may walk away from christianity precisely because we are content with our lives on earth, never considering that our very lives may be taken away within a blink of an eye. In fact, a friend of mine just had a bike accident and is now wrapped in bandages.
There can be a myraid of excuses, but definitely no reasons. If your experiences with the bible are only superficial, do take the first step to find out more. It'll definitely be beneficial in the long run.