Are there really people that honestly believe the Earth is... Or is it just someone seeking attention by being in some cult?
no there are actually people who believe all these things.
why?
Unfortunately Humans aren't as smart as people think. The brain is barely able to perceive the world properly. We like to make stuff up to help us feel happy.
Also being part of a 'collective' is in the DNA of Humans, unfortunately the idea's that bring the groups together can be anything. Why would anyone join a suicide cult? Well because others are. Why do people believe X? Because other people do. This has been proven many times, all it takes to start something like this is a small group of people making a joke. Or a singular well-spoken person who has mental deficiencies.