Try being in their shoes: you set up an experiment to prove the earth is not flat and is indeed round, you run the experiment and at the end you realise your experiment fail and would prove the earth is flat. Would you changwe your point of view ? I know I wouldn’t, I would immediately assume I am an idiot and messed up the experiment, because I am so much convinced it is round, it would be much more realistic to assume I messed up.