Alternative hypothesis that also fits the evidence: You look bad if reality turns out worse than your forecast. You look less bad if reality turns out better than your forecast, thus there is a built in incentive to make forecasts more pessimistic than is justified.

Imagine if 10in were forecast and then NYC actually got 2 feet. Then everyone would be complaining that the city wasn't adequately prepared for the 14 extra inches they received and would blame the forecasters for all the economic consequences of not being prepared.