As long as you control half an island, the other half keeps raiding and attacking you, usually by infiltration attacks over ground that the invaders don't know but the defenders do. Since the island had been preparing for invasion for some time, even the best-made defensive line would have been vulnerable to attacks from tunnels and coastal night raids. It turns into a meat grinder that eats men and destroys morale, more like Vietnam than WWI trench warfare. The Japanese were masters of this type of warfare, and everyone fighting in the Pacific knew it. Banzai charges were rare and costly, infiltration attacks were often devastating and always terrifying. With hindsight, we can sharp-shoot the decision of past generals, but we do so with information unavailable to them. Having personally survived several infiltration attacks, I can sympathize.

Bear in mind that the Allies had no idea how many troops the Japanese had on the island, meaning that The Japanese couldn't supply them by ship, but they could reach them by plane, bringing in more troops. They had huge stockpiles of munitions on the island already, and they were growing food on the island (it was summer), and the IJA had no problem taking food from the Okinawan civilian population, whim they often considered to be not-Japanese and racially inferior. Since their objective was simply to kill as many invaders as possible (vice a traditional battlefield victory), a stalemate would have played to the Japanese strengths.