I'm not going to defend flat earthers but I think it's important to understand where they are coming from:
Part of their philosophy (maybe not for all but still) is to put earth back into the center where it always was. In other words: they don't want to accept that earth is just one of many planets. Earth is the world and everything else are just lights on the sky. You can't deny that Mars is a round celestial body but you can deny earth is. So earth is still special and so am I and so are you.
It's based on the Bible for a lot of them. The Bible says the Earth has four pillars and that you can see all the nations of the world from a mountaintop.
I don't think you can separate these easily. Many Christian academics will point out that the bible doesn't really care about the shape of the earth but just reflects what people in the area thought. Same with the creation. Scholars will point out that the bible isn't consistent in the narrative of creation so these stories have a different, non historical meaning, explaining how the world is structured and how it works right now.
On the other hand, if you believe that God created earth in 6 days and made it special, the other planets can't be equal to us. Something the text doesn't really care about. It doesn't care about Mars or the Sun or the Moon, it just mocks the polytheists around that believe they are gods.
I mean there is alot in the bible you can take out of context and claim important. Why do people single out the flat earth and creation in 6 days? And historically, people had pseudo scientific theories focused on other parts like the 3 sons of Noah originating 3 different kinds of modern day peoples. There is a lot more and a lot to learn from why people choose to focus on this or that.
And why does this "someone else" focus on them? Because of the worldview that earth is special and not one of many planets. And that humans are special and not just one of many species
They're singled out because the Genesis narrative is, in the eyes of fundies, absolutely required for their religion to be necessary. If it isn't true, then there was no Garden of Eden and therefore no Original Sin. If there is no Original Sin, then humans are not doomed to be tortured forever by God by default and do not need God to sacrifice himself to himself to convince himself to not torture us forever because of our forefathers' mistake.