One advantage of the Earth being flat is that just like Stealth planes the relative reflection of rays is mostly directed away from the observer (unless they're looking directly from above), which subsequently decrease the chance of observers on other planets being able to "see" Earth from afar. If they don't know we're here they're less like to come and assimilate us. God*, in his** infinite wisdom knew how to protect Man from the rest of the Universe he created.
* other deities may be available
** her, their, other gender non-specific deity relevant pronoun