Descartes aimed for an objective science, not the science of the scholastics.  And that meant, especially, the deletion of final cause from science: “The entire class of causes which people customarily derive from a thing’s ‘end,’ I judge to be utterly useless in Physics.”

“Cause” is reduced to efficient cause.  Purposes and ends might be nice for morals and literature.  But they are not scientific, not knowledge strictly speaking.