Because they don't decide this on their own. A smart CEO gets board approval before going on a hiring spree, so afterwards, if the CFO shows a negative forecast

Because they don't decide this on their own. A smart CEO gets board approval before going on a hiring spree, so afterwards, if the CFO shows a negative forecast, everyone can go, 'Oh, who could have seen this?' CEOs get fired if the board loses trust in their abilities.