There are many hundreds of contexts in which prayer to the dead could have been mentioned in scripture and the early patristic literature. Instead, it's absent and sometimes contradicted.
Roman Catholics and Eastern Orthodox will often try to make the practice seem more credible by citing references to prayers for the dead, prayers by the dead for us, prayer or alleged prayer to angels, the presence of angels or deceased humans among humans on earth, etc. But a person can pray for the dead, believe in guardian angels, or think that deceased humans sometimes intercede for people or events on earth, yet not believe in prayer to the dead.
I recently discussed the example of Origen, who held some such beliefs while, at the same time, saying that we should pray only to God and not to lesser beings. In our day, Protestants believe that angels are involved in human affairs, and thus sometimes know about our prayers or are active in God's answering of prayer, without believing in prayer to angels or prayer to the deceased. Even in a mostly Protestant nation like the United States, many people believe in concepts like guardian angels and knowledge of earthly individuals and events on the part of the deceased, yet don't believe in prayer to angels or prayer to the dead. In past articles, such as here, I've discussed why such categories should be distinguished from one another. Even if we believed that an angel or deceased human is sometimes nearby or aware of what's happening in our lives, it doesn't follow that he's always present or always aware or that we can pray to him.
We also need to be discerning about the nature of the sources that are cited, such as their dating and authorship. Catholics and Orthodox often cite late, forged, apocryphal, and heretical documents to make their beliefs seem more historically credible than they actually are. See, for example, the work Turretinfan has done to demonstrate the poor quality of some of the sources cited by the Catholic apologist Steve Ray. In a recent discussion at Beggars All, the Orthodox poster Lvka failed to demonstrate any Biblical or early patristic support for prayers to the dead, but he did appeal to some beliefs in post-Biblical Judaism and offered a highly dubious and anachronistic interpretation of a passage in Justin Martyr. Just recently at this blog, a Catholic poster by the name of Christine posted a quote from Methodius that comes from a document that most likely is inauthentic.
Think about the large amount of evidence we see for prayers to the dead in modern Catholicism and Orthodoxy. It's prominent in their church services, their conversations, their television programs, their books, their web sites, etc. Contrast that with the absence and contradictions of the practice in scripture and early patristic church history. Why do Catholics and Orthodox so often resort to changing the subject to something like prayers for the dead or rely on so many late, forged, apocryphal, and heretical sources?