Even if you haven't had a spill, every five years you still get to update your look to the newest techno-color helmet trend. People have died after bumping their heads on a door, on the way to the bathroom in the middle of the night. It's cool to wear a riding helmet because somebody loves you. After your next ride, put your horse away, take off your helmet, comb your hair, and go find somebody you love. It's cool to wear a riding helmet because you can feel confident that your head is well-protected in case of an accident.
Nobody wants to fall.
But anyone who rides horses very much will fall from time to time. Head injuries are the most common reason for horse-related hospital admissions and deaths. It's cool to wear a riding helmet because riding helmets work. A bicycle helmet will not adequately protect your head in a fall from a horse. People fall off bikes and horses at different angles and in different directions.