With landscaping in general (atistic, or 3D) ground plane, however flat, IRL is usuallty broken up by vegetation and rolling hills, rock etc, or in streetscapes by various street furniture, poles, benches, curbs, trashcans, card, buildings etc etc.
 With water, there's usually a bit of haze or reduced visibility limiting this effect.

Also, when we are looking at our surroundings, our eye is sort of trained not to notice the flatness, and it only becomes more obvious when looking at a static imahe (photo or rendering).
This is just why many landscape artists thend to avoid long distant vanishing planes, and try to obscure them with props.

In 3D you can minimize this effect in several ways:

One of the more basic and less time and system resource taxing ways is to use more elaborate textures with some iregular bump, or even few rolling hills in it,  and atmospheric effects where distant planes vanish and melt into the horizon color (haze and fog).

Another way is to add more props like grass, bushes, trees, whatever is an appropriate add-on that will block good portion of the flatness, and not detract from the scene too much. (that's where finer points of knowing how to make a composition come into play). This can also be more taxing on your time and computer resources.

Also, there's postwork, where some of the effects I described above can be achieved in postwork.

Hope that helps a little :)

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