I’m going to answer this and go beyond and explain a lot more.
Flat earth believers don’t fly to the underside of the earth and take pictures because, they say, there’s a dome over the whole thing, which is why we have a “wall of ice” to prevent us from getting out and walking over to touch it. It’s a convenient idea to avoid various dilemmas like the one you pose: flying over the edge to look at the other side. Here’s an image that illustrates their belief, showing the Earth, the dome, and the movement of the sun.
Please note, in this image below, the shape of Australia.
Notice, in this image, the sun is over Australia. This is a picture of what flat earth people think Australia looks like:
Let’s compare this image to actual distances. Below you can see Australia is actually 2,511 miles wide. You can drive it. There are roads.
Now in the image below you can see that Australia is almost as tall as it is wide using actual measurable distances.
So flat earth people believe Australia is much wider than tall, which is the only way they can depict Australia on their flat earth map. Below is what Australia actually looks like:
One of a billion problems with flat earth maps is they cannot account for the distortions that they see. This can only be rectified by using the spheroidal earth model.
So there you have it. The real reason they can’t fly underneath to see the flat Earth is that the Earth isn’t flat!