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Why does erosion usually lower mountains over time?

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A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

Mountains are primarily created by tectonic processes that uplift the Earth's crust, but they are simultaneously subjected to erosion, which acts to reduce their elevation over time.1

Erosion lowers mountains through the continuous removal of surface material, including rock and soil, by natural agencies such as water, wind, and ice. This process involves several mechanisms:2

Weathering and Transport: Rocks are first disintegrated or altered by weathering. These materials are then transported away from the mountain slopes by rivers, glaciers, or wind, effectively wearing down the landform.2
Glacial Action: Glaciers shape mountains by grinding over the surface (abrasion) and plucking rock from the ground (quarrying), which deepens valleys and carves peaks.23
Fluvial Processes: Moving water in rivers scours the landscape, particularly during floods, carrying away sediment and deepening valleys to elevations well below the surrounding peaks.23

While tectonic uplift can counteract these effects, erosion remains a persistent force that eventually reduces the height of mountain ranges, sometimes leaving behind only residual mountains after millions of years.41