They are thinking critically, they just don't have the same assumptions. One could spend a long time arguing back and forth on whether or not the Earth rests on the backs of 4 elephants sitting on a turtle. The argument isn't important. What you should try to understand is why they are arguing for it on both sides. The proponent of the turtle theory is professing their world view as expressed by their religious leaders. There are benefits and detriments to having a society believe this. It gives people a common mythology and builds social cohesion. If they believe in the turtle theory, they might also believe in the other religious aspects such as loving and respecting each other. The flat Earthers similarly are professing their believes. Beliefs are a religion. They are building social cohesion based on this belief that others refute. They are forming these believes because the World has been lying to them on so many things. The Scientists have lost credibility in their eyes due to their corruption and subservience to corrupt elites. When you espouse reasonable arguments sourced from scientific theory this hardens their resolve. They spend copious efforts to try and refute your claims. Again they have very different assumptions and your differences will never be resolved but it is incorrect to say they are unwilling to put in the work. If they accept what you say and accept scientific reasoning, what do they gain? They lose the social cohesion of their fellow Flat Earthers. They lose the sense of belonging they have found. To believe in Science you don't understand (referring to them not you), is to believe in nothing. Understand the man not the problem and you will answer your questions in a more meaningful way.

except I didn't enspouse reasonable argument sourced from scientific theory. I asked them why the same constellations look different in the southern hemisphere. and received no response. I studiously avoided any non-personally verifiable theory for the very reason you mention. at least until much later when it became clear that they weren't going to talk about basic observational data anyway. It is better to lose the social cohesion of mistaken viewpoints and acccept personal responsibility for developing a viewpoint of the world that is coherent with observational data, then it is to just languish in cultish agreement for the sake of belonging.

iow it's not just that they're "thinking critically from different assumptions." The viewpoint doesn't explain basic observational fact.

Why is it better to lose the social cohesion of mistaken viewpoints? What tangible benefits do you receive? Also, a man who lived through Covid must acknowledge that the cultish agreement for the sake of belonging still exists under the guise of Science.

oh I argue with those people too it's okay to have smaller, more fractured groups that identify by niche interests. smaller groups that compete or cooperate on the basis of *the effectiveness of their ideas* is a better social outcome than large groups driven by herd mentality. but this is maximally effective if people are actually thinking critically and creating views that map to their observations.

I think to have success with changing these people's minds, one would need to offer a framing of tangible benefits. Like for instance if they were to attempt navigating the ocean by the stars.

since the benefits are primarily social as you pointed out I think that's unlikely to be effective. like covid hysteria, it's mostly just flag waving and group identification.