Quote from MTE:
Basically, the reason, as Ursa has pointed out, is volatility crush/drop after the announcement. When you buy a straddle coming into an event the IV is inflated so you're buying expensive options.
In other words, when you buy a straddle what you're really doing is betting that the subsequent move (i.e. realized volatility) will be bigger than the expected one (i.e. implied volatility). Sure some will make dramatic moves, but some won't move at all while others will move, but not enough to offset the cost of the straddle. Thus, overall, after taking into account bid/ask spread and commissions you'd be lucky to breakeven.