I wonder if it could have anything at all to do with continents being largely sites of net erosion, what with being up high?
Right at this moment we’re having a bit of erosion and deposition here locally! It’s raining, and dirt is getting carried down to, say, Red Lake. That alleged lake experiences this a couple of times a year, and gains a little-bitty mini-stratum of red dirt every time. Then the water evaporates and leaves a little crust of gypsum on top. Just like the dirt/gypsum couplets for who knows how many feet beneath. A lot like the red rock/gypsum layers a few miles away that are fifty or five thousand feet below the surface! Hmmmm!