Ancient man spent a lot of time around sea shores and lake shores. Spent hours at a time being forced to contemplate the horizon. They had direct visual confirmation that the surface they were on was curved. Boats sailed (and still sail if you can pay attention long enough) visibly over the horizon. This is probably what motivated Eratosthenes to try to work out what the actual distance around the Earth was, and he did it in the 3rd century BCE.
Modern man, wrongly informed that ancient man thought the world was flat; and especially modern religious zealots already prepared to discard all of science in exchange for heavenly salvation (or at least willing to sacrifice evolution) finds it quite easy to never notice the curvature of the Earth that he still can see if he simply looks. You don’t question that the Earth is round when you watch the people who will bring back your dinner tonight sail over the horizon to catch fish. The curvature of the Earth is as evident as the gnawing hunger in your belly. Modern man has no need to accept reality because society provides for all his wants while he egotistically pretends that what he believes is real.
That is why they call it “wilful ignorance”.