There's a simple biological explanation for why Smurfette is the only female Smurf, and that's that they're a eusocial species that is always born male, but one of them becomes female whenever there are no female Smurfs around. In light of this, Gargamel's efforts to kidnap her may be less an exercise in malice or greed and more an exercise in detached curiosity. Through the eyes of the Smurfs he may appear to be an evil wizard, but a biologist experimenting with the social dynamics of the colony with insufficient ethical oversight would seem no less terrifying and supernatural to a colony of laboratory animals. The fact that the Smurfs are always able to defeat his plots neatly within the span of a children's television episode is far more likely by design than random chance. Gargamel knows that keeping her away from the rest of the colony for too long would cause a new queen to develop, but in eusocial species that process often involves multiple potential queens fighting to the death until only one remains, possibly with many other casualties among male Smurfs bonded to them. Thus, he allows them to rescue her to restore an illusion of control that prevents them from becoming too distressed to accurately model the behavior of Smurfs in the wild.