I think you're looking for a more rational explanation with the OP than you're likely to find in a response. I think if I were to try to explain it using that school of thought, it would be mostly that people - and their bodies, after death - are symbols in addition to being merely themselves. This is something that usually gets brought up in reference to the president in newbie government classes, but I'd say it trickles down to "normal" people for those that care about them, too. In a spiritual/religious sense people aren't really their bodies by most definitions, but even after they've breathed their last breath there is still power in the body to people who cared about the person in life. I think the whoever mentioned the closure process was right on the money in that regard; seeing the body after death and getting that chance to say goodbye and accept a person's death, and to grieve with others at the funeral, has a very real and impactful psychological effect on survivors. It's "about" the person who died, but it's for the survivors, I think.
What it boils down to, though, is that deaths are traumatic to all parties involved, and as such tend to be something treated with more emotion than rationality. That's OK sometimes. People have all sorts of funny ways of dealing with death. Most never really want to let go of people they care about, and struggling to do so and move on is just part of the process for most people. Again, more about the survivors. Can you imagine the backlash surviving friends and family would have if people actually did some of the ridiculous shit mentioned in this topic to the dead? There'd end up with more than one body to worry about. I guarantee it.
In regards to wills, as mentioned it's already a legal contract and would be incredibly easy to dodge if laws were changed to prevent them anyway, barring a fairly major overhaul about transfer of possessions. There's already a cap on monetary inheritance, and frankly I think those who earn wealth deserve to see how it is distributed after they pass on - it's not like they couldn't have done so in life.
EDIT: To extrapolate a bit, there is an almost overwhelming amount of posts in Cong lately by people who are trying to follow rationalization and logic to an extreme at the expense of practicality. I think you'd find in trying to employ it that practicality will trump that sort of naïve idealism almost every time. There is little that is functionally rational about any extreme.