For many the "flat earth" is THEIR red pill moment. Once you see that the earth is not a ball, then so many other lies crumble. So, of course so many people argue about it.

And on the other side "Those silly people of old believed the world was flat, we are smarter than them" is the lie that allows them to think they are smarter. So it must be defended. So argument ensues.

It is similar to steel beams do not melt in kerosene or paper fires... argument ensues.