Why not train for an all engine out forced landing as well?
Because there has been several all engines out forced landings, that's why. As a simulator instructor I have observed many of these over the years. In just about every case the pilot would have crashed either because he was grossly overshooting or grossly undershooting. Most crews we observed needed at least three practice forced landings from 15,000 ft before they got the hang of glide control with various stages of flaps. The B767 captain of Gimli glider fame did an exceptional job of forced landing all flaps up on a 7000 ft runway. Afterwards he said if only Air Canada had given him just one practice simulator forced landing from height he would have been much more confident when he did he real one.
So much valuable simulator training is wasted by a syllabus or check pilots that insists on lengthy checklists, lengthy taxying and lengthy briefings that take up simulator time when this time could have been much more productive such as practicing dead stick forced landings and final approach ditching on a simulated black night over the simulated ocean. There is real instrument flying skill needed to set an airliner down in the ocean. With the plethora of airline crews whose manual raw data instrument flying flying skills are seriously degraded by company imposed policy on full automation from lift off to touch down, there would be no hope for all the souls aboard.
You can argue all day until the cows come home about the statistical improbability of a dead stick landing or a controlled ditching ever happening to each one of us. But it would be criminal if the captain of an airliner was so incompetent that he could not pull off a successful dead stick landing or a ditching if such an event occurred. We have multi million dollar full flight simulators to ensure we can deal successfully with such events.
Even the cabin crew are required to be competent in dinghy drill which suggests the possibility does exist an aircraft may have to ditch one day.