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.