Because the profit comes over a shorter period, i.e. shorting stock from the 3rd to the 5th. If they just bought at the lower price after the announcement they

Because the profit comes over a shorter period, i.e. shorting stock from the 3rd to the 5th. If they just bought at the lower price after the announcement they would have to wait for a longer period before they would gain the same profit as through shorting the stock.