There are various reasons but the central one is a defective view of Christianity. This has its roots in the Pietist movement under the leadership of P.J. Spener in the 17th century. Pietism began as a healthy protest against formalism and a too abstract Christianity. The totality of human existence was not afforded a proper place. In particular it neglected the intellectual dimension of Christianity. .
True spirituality covers all of reality.
There are things the Bible tells us as absolutes which are sinful - which do not conform to the character of God. But aside from these the Lordship of Christ covers ALL of life and ALL of life equally. It is not only that true spirituality covers all of life, but it covers all parts of the spectrum of life equally. In this sense there is nothing concerning reality that is not spiritual. . Christianity is not just a series of truths but Truth - Truth about all of reality.