This thread brings up a couple thought provoking topics.
Someone asked what FEr's would talk about if no REr's came here. Just look in the FE Believers section, it looks like they wouldn't talk about much. Some of them post long monologues there but I suspect even those would dwindle down if there was no REr's against which to make arguments.
But back to the more direct question, why do round earther's bother showing up and interacting -- this is naturally a question I asked myself many times.
For me personally, it started when a friend said to me "Hey the earth is flat! Gravity does not exist!" of course I was like "Of course it's not flat, of course gravity exists."
Then I realized I had no direct evidence to prove him incorrect.
So I wanted to check into it more. Being an inquisitive person, I asked him more questions.
He had no answers, but referred me to Mark Sargent, so I watched hundreds of hours of mark sargent videos over several months, trying to find answers. He didn't seem to have a lot of answers and teaches the idea that everything we see in the sky is just a 3D projection such as the sun, moon, planets, and stars.
So I began trying to learn as much as I could bout FE and meta-FE (i.e. what the belief was and how seriously the believers believed it themselves.)
I also began trying different experiments.
I performed the cavendish experiment, and found there to be an attractive force between lead weights which was not magnetic and not electric. It was in the ballpark for gravity.
I also got a theodolite (like what surveyors use) and measured some mountains and found that the height of a mountain drops off non-linearly with distance -- my measurements came out quite close to 8 inches per mile squared, which is a good approximation for the generally accepted shape of the generally spherical earth theory.
I continue to perform experiments and learn as much as I can about the topic.
At the same time, I care about people and it makes me sad if they are believing a lie and shutting out their friends because their friends don't also believe the earth is flat.
In fact my friend who introduced me to flat earth was very unhappy with me when I showed him how I'd measured the height of a distant mountain with a theodolite and got a reading that fit the globe earth theory.
So I come here to help potential flat earther's see that there are problems with the theory to prevent them from falling hook line and sinker, and to also help flat earther's see that there's problems so they aren't so fanatic and maybe work them back to a better understanding of observable reality.
At this point, I'm finding that the majority of the evidence points to globe earth, and that flat earth claims are mostly incoherent.
I've also found that a lot of the highly experienced flat earthers seem afraid of the truth, afraid that if they look at all the evidence they may have to abandon their theory.
And look. If you'd measured the height of a distant mountain with a real surveyor's theodolite, and it agreed with your world-shape-view, wouldn't you want to tell people who believed it was a different shape?
And look at it the other way - if someone measured and found the world to be a shape different than you believed, wouldn't you want to know?
I sure would want to know if I was wrong!
I couldn't care less what shape the world is. As long as I correctly know what shape it is.
If my measurements show it to be flat, I'll be the happiest flat earther there is. If they show it to be a globe, again, I'll be the happiest glober there is.
But for now, I'm still on the search for the single very best evidence of a flat earth I can find! So far I have not found even one good one!