Why is there so much of the earth’s surface at or near sea-level?
According to the hypsometric curve (plot between elevation and % of earth's surface), only about 29% of Earth's surface is above the mean sea level, rest 71% has an elevation less than mean sea level. There are a number of reasons for this observation. Erosion of land mass by water, wind and other forces reduces the elevation of continental features. The eroded material finds its way to the bottom of oceans because of gravity (from higher elevation to lower elevation) and this reduces the difference between the elevations of ocean and continents. An example of this is the average height of young and old mountains. It is often reported that older mountains have lower elevation as compared to younger mountains, due to erosion and transport of material. Interestingly, the tallest mountain is only about 8,848 m tall, while the deepest point is about 14 km underground. Also, oceans cover 71% of Earth's surface and acting as a single body, the ocean water is at a (more or less) constant level. Hence, most of the Earth's surface has low elevation and is close to sea level.
Hope this helps.