I thought this was such a great question that it's the reason I finally signed up to be able to post. I'm on the fence as to whether spirit communication is actually spirit communication, psychic communication, or some other, more mundane phenomenon. Although I do think it is a real phenomenon of some kind.
The only reasons I can think of why these questions have not been asked is that the communication tends to be tenuous, passive, and loaded with assumptions. First of all, I think most mediums are involved in a culture where they already assume a spirit world exists, and where heaven or God are less relevant concepts, so they never bother to ask those questions. Secondly, they often seem to have limited control over the information that gets passed through. The answers to such important questions are likely to be complex and require long, philosophical explanations, whereas the answers that get through in the form of spirit communication are often terse, unreliable and vague. Think of it like two people who speak two different languages. They may be able to communicate on a superficial level through universal symbols, emotions and gestures, but they could never have a long, important, abstract discussion with these limited tools.
Having said all that, the closest thing I've seen to getting some actual answers to important questions is contained in the the Scole experiments (see video below). Seems to me if more people took these things seriously, one day we may be able to ask those questions. Either that, or it's all a dead end (pun intended).