One of the great benefits of boards for flooring and siding is that they puzzle around things easily, if you make a bad cut you have only messed up a small amount of wood and usually that board can be salvaged and used somewhere else. The tongue and groove allows them to interlock. A 4X8' sheet of plywood does not interlock with other sheets, is pretty heavy and cumbersome and can easily damage finished areas as you wrestle it around corners etc. The most difficult part is measuring, cutting, mitering and compound mitering all the jogs around doorways, alcove walls, stairways etc. And if a wall isn't perfectly plumb then making the join at the corner requires scribing, cutting along that line with a skill saw and guide, or ripping diagonal cuts on a table saw. Total pain in the ass. I believe only the most skilled carpenters can make it look good.