Why can’t it be amended? My region has pretty shitty clay soil but I’m able to mix in compost, mulch, and other amendments and grow all sorts of tropical fruits and other plants that shouldn’t grow here. Am I missing some reason why the farmers have to rely on topsoil?

What do you mean, why do we have to rely on topsoil? That's what almost all food grows in. What else would you effectively grow food in? What you're doing by adding compost is basically creating topsoil yourself. Amending can only go so far. In some cases it's literally just been degraded away like when deforestation causes land to be more susceptible to erosion by flood or landslide. You may be able to make soil amendments work while you have fertilizer and added nutrients derived from fossil fuels, but once those aren't economical, you're getting out what you put in naturally, which is a lot less than modern industry has acclimated us to, and probably a lot less than in historical times if the topsoil quality was drained of nutrients that can no longer be replaced.