I believe because it is what my eyes tell me to believe. Now you will say "But RE predicts that the Earth will appear flat too! You can't always trust your eyes!" with which I partially agree. So when sufficient evidence is presented to me that my eyes are in fact deceiving me in this matter I will become a REer for life. But after a great deal of time on this forum, having REers daily attempt to prove their ridiculous hypothesis in every conceivable way, I have yet to see that evidence. So I have no reason to believe my eyes are deceiving me, and continue to resist the bias that has turned many an otherwise reasonable and intelligent soul into a raving unwitting conspiracy agent insisting that the Earth is round on a web forum dedicated to FE ideals for no reason, I can imagine, other than the desire to be an instigator. The Conspiracy has trained you all well.
The terrible thing is, I was once one of you. I too came to this forum sure that the Earth was round with no concrete reason for it. It wasn't until I spent some time here that I saw the light of truth for what it was, saw how ridiculous and (frankly) sad it is to cling to such an unreasonable notion for no other reason than that that's what NASA, my teachers, my parents, the government, etc have always told me. I understand the desire to be led; after all, why think for yourself when you can have people in authority think for you? Life's easier that way, huh? But just the same it is my sincere hope that given time you, too, will come to the correct conclusions; however, my faith in humanity isn't strong enough for me to truly expect it of you. So continue living in miserable ignorance and enjoy making fun of those clearly far more enlightened than you.