Probably because he wanted to film there.
It was fiction. The Hitchcock movie The Birds is based on a short story by Daphne du Maurier(story) published in 1952 and set in Cornwall, England. The Wikipedia article mentions what gave the author the idea.
The story itself is a good creepy read. A lot of du Maurier’s fiction actually holds up well over time.
The structure of the Wikipedia link interferes with direct access because of the parenthetical at the end, but just click the ”(story)” link referring to du Maurier on the disambiguation page.