I guess a lot of these flat earthers are trolls, just doing it to get their kicks, but there is a certain number who are serious about it. They're impervious to reason, and angrily insist they're right, insulting and threatening those who provide them with reasonable explanations, such as yours above. Then there are those who are really just too stupid (and I mean that) to see how foolish the idea of a flat earth is. I belong to some astronomy-based Facebook groups where this topic comes up over and over again, despite all the evidence against it. The phenomenon, and the reaction to it, would make for an interesting psychological study in its own right.
I think we're now seeing the effects of years of dumbed-down education and lack of critical thinking when we've reached a point in the 21st century where the shape of the earth is being called into question.
Good luck in any future efforts in trying to reason with a flattie!