It's because it's a heavily advertised (i.e. funded) psyop.

There are no flat earthers; just agents and products of the psyop.

They are contrived to sound so offensively stupid so you won't ever seriously consider, discuss, or research the subject because of how valuable it is.

Imagine spending so much money on an ad campaign to make something (the idea that the world is flat) already ostensibly stupid by default seem even stupider. Highly curious, wouldn't you say?

Methinks the woman doth protest too much...