But remember that the Anglican Church and the various protestant factions within England, had been somewhat cut off from the central Roman Church since 1534; maybe the information was not available to Ussher.
In any event, Ussher's Chronology never had much impact on the rest of the world, but is still used in the English-speaking world, especially in the "Colonies".
Still, Ussher had made a valiant attempt to use the Biblical stories and whatever historical information was available to him. He did warn the reader that everything hinges on a proper determination of the date of the Flood.
Alas, the story of the Flood found in the Bible is now know to be a recounting of an older Sumerian story (picked up by the Jews from the Babylonians) which could, itself, be a recounting of an even older story.
The story of a global flood was even taken to be allegorical, as most scientists and historian agreed that there might have never been such an event. maybe it was a local event (like a large meteorite falling into the Sea of Galilee or some other large expanse of water) or the opening of the Strait of Gibraltar.
In any case, not finding any evidence of a global flood in old Egyptian records was good evidence that the Flood as described in the Bible did not exist.
Recently, though, we are beginning to find evidence that there COULD have been a global flood somewhere around 10,000 BC to 12,000 BC. There is some evidence in the North American Arctic regions and in East European Arctic regions. Still scant evidence, but worth looking at.
As to why there are people who believe this, it is because they are told to believe it.
Consider that recently, in a country that used to be regarded as civilized, a leader was asked to produce his birth certificate (over three years ago) and he did. Intelligent people knew that this should be the end of the discussion.
Yet, some a88h0le decided to created doubt about this, knowing that in that particular country, it is easy to fool some people into believing anything.