The weird thing about mounds is that they were used for a whole lot more than just burying the dead. Think of them as a style of construction that was used for many things the builders of them felt justified construction of them. I think the best example of this can be seen across Eastern North America, from Spiro to the Marching Bear group we see that mounds were often the place of feasting events. It may be less clear if the feasting was associated with the builders of them or with some return to the mounds seasonally, but in either instance we see that something was occuring on a community level. I think this is the best way to look at any mound, burial or otherwise. These were a part of these communities, they had a meaning that we today may not be able to accurately describe to a full extent. Even the smallest of mounds would have taken many hours to build, and at places like Monk's Mound we are seeing the work of hundreds or even thousands of people. We can get into the spiritualism of what mounds meant, but I think the best answer is that these structures were something that everyone in a community would have felt some attachment too. They were important enough to denote resources to build, and with that they were important enough to hold feasts and in the case of burials, inter human remains. It's more of a case that the burial of community members would have been done in a manner the community felt was important and significant, the construction of a mound.