Several reasons. First, it is an act of humility to appeal to another person for help. Do people just walk right up to the Queen of England and ask for help? No, they usually go through someone close to the Queen for a favor.
Do you think the Queen of England wants to have a personal relationship with you? Do you think God does? I think this is important. If you believe that God really desires to put dead people as "buffers" between Himself and His living children, then you believe in a much more impersonal God. I believe that our God wants us to trust Him completely for everything, and therefore come DIRECTLY to Him. God has an open door policy.
How much does God want us to communicate with Him? Certainly not as much if He prefers that we go through middle management with our (intercessory) prayers. That is not communication with God, it is communication with dead people. This is completely different from communicating with a living friend about prayer. The Bible is clear that this is common and proper. However, the Bible does NOT teach that it is common and proper to seek out the dead.
God has placed other people in our lives for the reason of bringing us closer to Him. God formed a Church, a society which we all are helping each other to achieve fruitation. Just as in a human family, we often rely on each other for help.
Sure, and that is fine. But what do all of these people you speak of have in common that dead people do not have? A pulse. All of the people who have ever helped me have communicated to me. No dead person ever has. Why do you seek the dead when the living are all around you? You indicated in your other post that it was because a dead saint will pray for God's will, and therefore, there is a greater chance that it will be granted. Why not just tell all of your living friends to also pray for God's will? Does it really change anything anyway? God's will is God's will no matter who prays for it, right? It's not going to help you get whatever you originally wanted if it's outside of God's will, is it?
First of all, Mary and the saints in heaven are not dead. NOTHING can separate us from the Love of God, not even death, as Paul tells us in Romans 8. Secondly, God created people and situations in our lives (Providence) to specifically bring us closer to Him. God is invisible and incomprehensible. People are not. This is why Christ became incarnate - to reveal God to us through the Mediator. But this doesn't mean that God no longer sends us further aid to come to Him. He helps us daily - DESPITE that fact that HE has already sent the Mediator.
How much does God want us to communicate with Him? Certainly not as much if He prefers that we go through middle management with our (intercessory) prayers.
Some people have a close connection with another person that has suffered similarly as they have. For example, patron saints. Say a person was blind. Was Jesus Christ blind? No. But what about some of the saints? There WERE blind saints, those who were able to put God first in their lives - despite their blindness. Thus, by emulating a particular patron saint, a person comes closer to God through another creation that God made for the example of others.
You need to stop thinking of the saints and Mary as competition with God. HE MADE THEM WHAT THEY ARE/WERE!
Regards