Or why do people here always ask "What does the Bible say?"... why not ask the proper "What does God say?".
I hope it's obvious that the Bible (a book) and God (a divine being) are not the same thing. It has never been part of Christian theology that the Bible should be worshipped, or that believing the Bible is synonymous with Christianity. The Bible as we know it today did not exist at all for many decades after Jesus' death, and in its present form for hundreds of years, yet Christians certainly existed during that time. Almost all Christians take the Bible as a true and infallible word from God. But views on other communication from God differ. One area of difference is in Jesus statements about what is to happen after his death. He never states that his followers are to use the scriptures only, but "I will be with you" and "My spirit I give to you". Interpretations of that differ.
One view is that the scriptures are foundational, but the Spirit gives additional revelation to Christians. In the Catholic view, the Spirit does this through the Church, specifically the church leadership. In other traditions this can happen through the church in community: in still others, to individual people. In either case revelations though the Spirit cannot contradict the Scriptures, but they can add to them. Some denominations believe in the office of "a prophet", who can convey commands from God for a specific time and place. Except for in some small denominations, and some specific cases in Catholicism, post-biblical revelations are not given the same 'infallibility' status that the Bible has.
In another view the presence of the Spirit to guide the followers of Jesus like this only lasted for the Apostolic Age, and was replaced by the Bible. This is the view of much non-Charismatic Protestantism. For those people the question "what does God say" and "what does the Bible say" are essentially synonymous.
However even the most ardent believer in a solely biblical authority will usually allow for the idea of personal guidance - i.e. that a person can pray to God and ask what to do in their specific circumstances, and receive an answer. In this case that is "what God says" to them. However what God says to one person doesn't necessarily apply to another.
The Bible is not God, but God breathed and God sustained.
People are asking, "What does the Bible say", because the underlying understanding is that we all know the Bible is real. However, the secular people visiting this website would/will argue the existence of God but not the existence of the Bible.
Some people also take the Bible to be different levels of true. Some, like myself, believe that God said everything in the Bible, others don't believe that the bible is God speaking the whole time.
This it is appropriate to just ask, "what does the Bible say".
It's arguable that one could say "What does the bible say?" and "What does God say?" are interchangeable since the Bible was inspired by the Holy Spirit, and is called by many the "Written word of God". By saying "What does the Bible say?" we provide a common point of reference between individuals who might hear from God differently or even interpret what He says differently.
The two are not the same, but we often ask "What does the Bible say", because we believe it's the best source for God's intentions.
The Reformed tradition even goes so far as to have Sola Scriptura as a core tenet. This basically means that they believe that everything God thought we needed to know is available in the Bible, and no further writings are necessary to live a Christian life (though obviously they see it as beneficial, as the Reformed churches publish a lot of books).
Other traditions may treat the Bible as less central, but it's still a core part of Christian belief.
While one is a book full of words and one is a divine being (and thus are in no way the same thing), Christians believe the Bible to be God's inspired word. Thus asking "What does the Bible say about x?" is approximately the same thing as asking, "What does God say about x"? The Bible is the written record we have of God's proclamations to men, and the primary way we know who He is and what He stands for.
The Bible is an anthology of books inspired by the Holy Spirit.
God's Word (Gn 1, Jn 1) is the second person of the Trinity.
God is His Word (Jesus). God's words are not His written words, but they ideas they express.
Unlike in Harry Potter 2, if you write a book, it doesn't become you. If you write a book you stay you, your thoughts go into the book but remain with you.
God's Word is His thought (Jesus) and the act of thinking that thought is the Holy Spirit (Their mutual love). You can't separate any of this from the one Divine Entity.
I'm not sure that I would have any general qualms about using the phrasings interchangably, but they do have the potential to give different connotations with regard to the mode through which a person is seeking his answers. This points to the distinction between revelation, inspiration, and illumination. (FTR, I'm not endorsing all of the implications put forth in that link, but it does do a good job summarizing the differences between the three modes of communication).
From the link:
Revelation: From God to man (man hears what God wants written)
Inspiration: From man to paper (man writes that which God wants written)
Illumination: From paper to heart (man receives that which God has written)
When we (directly) look to "what the Bible says" in order to (indirectly) discern "what God says," we're trusting in the inspiration of the biblical authors and illumination of the Spirit to "shed light on" those words of Scripture which have been revealed to those authors and offered to all men. If someone uses the phrase they're looking to "what God says," it can be ambiguous as to what exactly they mean (are they appealing to a "special revelation" of visions and prophecies that they alone are privvy to?)