Fair point, which is why black polythene wants putting down when soil is fully moist i.e. now. After that when it rains, there is usually enough moisture seeping through the holes made for each plant, and also the plastic retains more soil moisture. Better a little dry because if too wet, slugs abound.
The tubers develop near to the surface under the plastic and no earthing up is needed. Harvesting is simple because after removing the plastic in late summer, you can access most potatoes by simply pulling the stems of each plant.
Use old fashioned black polythene not the more porous Mypex, which may let enough light through to help bindweed and green potatoes, although I am not entirely sure of that.