It still mystifies me. For all possible psychological reasons mentioned and how conspiracy theories like that thrive - this one is most ridiculous one as even reptilians could somehow work. Video missed one thing as dude does mention that flat-earth believers probably do not have means to validate it but ... there is quite few rich celebrities amongst them so you'd assume they'd gather enough funds to send a satellite to space. Unlike reptilians... there is nothing to gain by whatever evil organisation there is if Earth was actually flat. Someone surely would have chucked Trump of the end of the planet by now if it was true.
It's not about proof, it's about man's dominion over others. "I say it is thus, therefore it is. It matters not if I am right, only that you yield to my authority when I declare 'truths' that serve my ego. If the facts do not serve my interests, then they are wrong, as are any who believe them."
You don't need expensive equipment to measure the Earth's curvature. I've done it with a $400 used surveyor's transit on my own property (admittedly a large property - timber land). The bubble level (smaller tube attached to the telecope) on a transit always tells you local horizontal, to great accuracy. When you measure the heights of two points nominally the same elevation that are far enough apart (say across a valley), they will *both* appear to be downhill from the other. That's because on a sphere, the ground curves away in all directions with distance, but the bubble level is always perpendicular to "down" locally (the center of the Earth).
Knowing that the Earth is curved is one of the corrections careful surveyors apply to their measurements. Another is accounting for stretch and temperature variations in steel measuring tapes. Such things don't matter for laying out a house foundation, but for larger things, like road construction, they do matter. To believe in a flat Earth is to believe that every land surveyor for the last 200 years is in on the conspiracy, because our entire system of property lines is based on a spherical Earth.
I just assumed they would somehow doubt any evaluation and equation unless they seen it with their very own eyes. Anything related to physics, cartography and even geology can tell them Earth is a sphere. I somehow went too far - literally - as you do not even need a satellite because ordinary plane would do. When the sky is clear you can actually see the land properly so I am clearly giving it too much credit and Jopsy here is right.