Why are there no subduction trenches on continental land?
Continental crust is thick and relatively buoyant.  Ocean crust is much thinner, and relatively dense.
Hence ocean crust &quot;goes down&quot; more easily, and the density can even help pull the ocean slab due to metamorphism.
In contrast, the continents are almost like floating scum which won't go down! When they collide, you get faulting and folding, and the building up of very thick continental crust. Tibet is the best example today. Yes the Tibetan Plateau is high, but the crust is over 100km thick! (think of an iceberg, most of it is below)