Why does the people of Amity Island forget that money isn't everything?

You have to understand that much of the action in Jaws revolves around the Fourth of July celebrations. This is the busiest time of the year in Amity, when people come from far and wide to visit the town and check out its golden beaches. There's a lot of money riding on a successful Fourth of July; local businesses need the infusion of cash that a large influx of tourists brings. So it's not surprising that local people are initially somewhat reluctant to accept that there's a fearsome great white shark stalking the coastline. They want to be absolutely sure that it really is a shark that's responsible for the recent spate of deaths before they even contemplate closing the beach and potentially damaging their livelihoods.