Because sciences are exact practices and language is not, translation is much more of an art than it is a science.
At first glance, translation may seem to be a science. There are formulas to languages, and to translate one into another may seem like purely substituting one word for its equivalent in another language. However, anyone fluent in multiple languages would say that doing so often fails to fully convey the original meaning.
Direct word-for-word translations can vary from clunky and awkward to nonsensical and incomprehensible. This is because languages contain subtleties, nuances, and culturally relevant expressions. As a result, properly translating between languages takes certain artistic creativity and intuition.
Furthermore, if translation were a science, there would not be so many different translations of works of literature. For instance, there are dozens of English translations of the Odyssey available. The reading of each version changes dramatically from one translation to the next because, at its heart, a translation is an interpretation.