The Earth is not perfectly round. Nor are the continental plates you’re talking about.
This is an exaggerated view of the shape of the Earth to show you how it’s not perfectly round:
This is a less exaggerated view of the Earth missing its oceans. The height of the continents is exaggerated:
But, because Earth is huge compared to a human and all human reference points, it would look like this to the naked eye, should all the water be removed:
This is a very zoomed in view of Earth showing its curve. This photo was taken by science students using a cheap weather balloon and a camera. You can do this at home:
Here is the farthest sight line possible on Earth. I’ve added a show line so you can see how at both ends the land begins to curve down:
The curve can typically not be seen by people standing on the ground because they are too close. But to help wrap your head around it, here are some pictures I just took:
You see how the surface looks flat beneath the flat tan line?
How about this, a more zoomed out image:
Do you see how the surface curves under the flat skewer?
And now you see from farther out, where your expanded view allows you to see the curve of the Earth.
You can easily do this on any transcontinental flight where they go up high enough you can see it with your own eyes from the window of a plane.