More bodies means a larger quantity of material to process. If you drop a sugar cube inside a drink, it will dissolve slower than having the same quantity of sugar as a finely ground powder.
For rapid and effective burning of any material you want is as great of a surface area to volume ratio as you can get but what you are proposing is increasing volume, so what do you think happens to the time required to cremate a body? You speak of the sanctity of the deceased being thrown aside for efficiency by piling more inside but that would actually make it go slower. Since the oven will always be generating heat around the body from the outside, piling more in will always result it either the exact same rate or slower. It will never accelerate from you dropping a larger quantity inside, especially when it comes to cremation since human bodies will contain a fairly significant quantity of water. Contrary to what Hollywood and self-appointed "historians" will have you believe, humans are not actually that easily flammable, the fat inside the human body is nowhere near enough to cause them to combust and cook themselves until ash.
In fact, the analogy regarding powdered sugar is a lot more relevant than you may think considering the fact that there are plenty of proposals by people that bodies could in the future, instead of being cremated, be dropped into liquid nitrogen, broken up through the use of vibration and then have the human flesh powder disposed of instead. Want to know why they would go through such hoops just to get rid of a dead body? Because that shit takes forever and there is no magic trick to make it go faster, what you have is effectively a fairly heavy bag of water and so people are looking to freezing and powderising them as a solution.