Because they're human, and humans have a hard time imagining how very different conditions can be.

I know someone who had a hard time raising basil, which mystified me. What could possibly be hard about raising basil? You put it in a pot on a windowsill and water it when it's looking a little limp and it grows.

My friend was living in Wales. Basil needs more sunlight than occurs naturally there.

Left to myself, I never would have believed that water boils at different temperatures in different places. It sounds like a practical joke, but there's good physics behind.