well, they are. Moses was dead but aware of Christ's Passion and spoke to Him about it at the Transfiguration, the rich man and Lazarus and Abraham were all aware of their states and the states of others (and Abraham knew of the Law and prophets long after his death), St Peter in his epistle speaks of continuing to work for the faithful even after his death, and the saints are aware and praying in the book of Revelation.
But it was Christ who invoked Moses, not a saint. He appeared on earth; did not remain in heaven (or I suppose you could interpret it as the disciples were transported to heaven), and he never spoke to anyone except for Christ.
In the Revelation, it was again a special case of souls in heaven pleading their own case to God, not pleading the case of others who had prayed to them. Further, the Greek can be interpreted as "screaming" rather than "talking", much like the blood of Abel (did the blood of Abel literally cry out?) Note that these were also martyrs, not just saints who had passed on of natural causes. To interpret this as consciousness is not completely unfounded, but still a little dubious given the nature of the Revelation and the fact that John was in heaven - a place bound to be much different than our current existence.
The story of Lazarus is just that, a story (a Parable), and it is always dangerous to deduce doctrine from a Parable that is secondary to the main point. But, if that's what we're going to do, note that Lazarus and the rich man never conversed with each other. It was strictly the rich man and Abraham (a special case, and both dead at the time), and Abraham denied the request to send someone to speak to the living.
I'm not sure I know the reference in Peter. Can you tell me more specifically, please?
So, all of these seem to be special cases with a special purpose. It seems to me a bit arrogant on our part to expect the same treatment as the Apostles ... and even then the Apostles were not given this special treatment because of who
they were but because of what
God intended to do. Just like the "big" miracles, if one takes an honest look at scripture one finds them to be very rare and for a very specific purpose of furthering God's will, not a mundane daily experience of every living saint.
I'm surprised no one brought up Samuel, but maybe that's because of the circumstances of Saul's wickedness under which that occurred. Probably not a good example.
However, with all that said, note what I said earlier about the Confessions. They concede that it might be possible that the dead saints pray in some general way. I just find it curious that they didn't mention the verses you quote. I'm quite sure they were aware of them.
So, next question: how is it determined who one can pray to?
