While the popular view is to buy the straddle a few weeks before earnings, you have to take into account the time decay leading up to it. Sure you buy low IV, but when the stock does a non-move, and IV comes back to levels which you bought at, don't forget time has passed since then. Unless IV spikes hard before earnings and in the same month your straddles in, directionally the market has priced the straddle correctly most of the time.