Why Would Someone Think That Psychology Is Not A Science?

Scientific inquiry relies on empirical data, that is, things that can be independently measured and quantified so they can be understood.  Psychology often is buried amongst layers and layers of cause and effect, so they are very difficult to measure, much less understand.

But one could argue Physics is considered a science, yet scientists don't understand all the intricacies of sub-atomic particles.  There are unknown areas in the knowledge of physics.  So when people try to describe things in terms they understand, they overlook the all the gaps they do not understand and still call it a 'science.'