Aye, this question
has been asked
several times before
... to quote myself:
The Reavers - the whole plot's raison d'Ítre - make no sense whatsoever. If they're rage-filled crazies who are always super-aggressive, as the movie pretty clearly states, than how can they pilot and maintain their ships? Particularly since the show and movie are always stressing Serenity's fragility, the Reavers' spacefaring capabilities should be impossible.
It didn't have to be that way. For all we know from "Serenity" (the pilot) and their attack on the moon in the movie's first half, Reavers are just normal guys who take stim-packs or chug Romulan ale before going into battle. But, in order to make a fairly banal statement about human nature (breaking news: doping the bejeezus out of people infringes upon their humanity!), Whedon absolutely shredded the already tenuous credibility of his 'verse according to his own rules.
's answer is probably the best in-universe reply we're going to get. My reply is that it shows that Whedon didn't spend any more time thinking about the plot of his movie than he did on sketching the characters therein.