It's a simple fact that there are dozens upon dozens of experiments out there demonstrating that we can "see too far" given the purported curvature of the earth. Multiple laser experiments, and many optical experiments with telescopes and Nikon P900/1000 cameras, etc. Refraction is the magic weapon to explain all of this, but several of these groups have done the refraction math and conclusively ruled it out. I even saw one group from South America who used precision / engineering GPS equipment to demonstrate that two tall buildings that should, on a globe earth, have been leaning away from one another (being miles apart), were actually parallel ... with their tops and their bottoms being exactly the same distance apart. There's hours of footage demonstrating this.
One guy who was anti-flat earth went out and demonstrated this with mountains that were about 30 miles apart (but with a simlar elevation, within like 50 feet) ... lined up perfectly and had the same perceived elevation, even though the one farther away should have been a few hundred feet lower. Refraction couldn't account for this, and at the end of his video he was seriously distraught and asking for people to come up with explanations for this, saying he had no idea what to make of it because he didn't want to accept flat earth. He was an active anti-flat-earther on various forums.