Why are there no subduction trenches on continental land?

As mentioned in another answer:

Continental crust is thick and relatively buoyant. Ocean crust is much thinner, and relatively dense.

Let's look at a simulation of what might happen. Here's a figure from "Continental collision and slab break-off: A comparison of 3-D numerical models with observations":

Let's look at this in detail.

You start by having a lithospheric plate consisting of oceanic and continental crust, going to the right, and forming a subduction zone under another lithosphere plate carrying continental crust:

Then the oceanic crust keeps going under into the mantle, eventually reaching a stage where both continental crusts are now in contact:

Finally, the oceanic crust tears or breaks off the continental crust part, and sinks into the mantle, with the surrounding asthenospheric mantle filling up the space. There's no dense oceanic crust to pull everything down to form a trench now, so instead you have mountain ranges and the two plates are amalgamated.