Why are the fastest teams so competitive in q1 and q2?

wouldn't it be wiser to just do one hot lap to save tyres? if vettel does a couple laps that will definitely keep him from getting knocked out, why waste the fuel and tyres if the times are discarded? also, why were some of the faster teams using the softer compounds? this should be expected from, say, a toro rosso or force india, buy why red bull or mclaren?