It’s rare but it happens, thus the seat belts.
I was on a flight from Barcelona to New York JFK about 15 years ago when we hit severe turbulence almost halfway between the coast of Portugal and Cape May, NJ—in other words, over the middle of the north Atlantic ocean. It was pretty scary.
Another area where severe turbulence happens often, like almost every day, is the front range of the Rockies, near Denver and Colorado Springs. Flights from the east to the Bay Area airports always seem to hit this.