Mainly because GPU throughput is increasing much faster than CPU throughput. Spending cycles removing triangles doesn't matter if the GPU can just churn through them like nothing. In the context of a moving camera you want to reuse as much data as possible between terrain updates. With regular nested grids a ton of data can be reused, only the height needs updating as you move the grids around (and height can be reused between adjacent grids half the time if they have a power of two resolution relationship).

But baking irregular meshes into a tree-structure could be powerful too I suppose, I'm not that familiar with algorithms involving them. All of this is highly dependant on actual size of terrain and LOD requirements, terrain rendering is a deep rabbit hole.