There's a lot of hidden costs and infrastructure needed for magical flying beasts. Not to mention limited capacity. Little can compete with the horse drawn cart when it comes to capacity for price though.
You would just not believe the regulatory burden of maintaining magical creature flying safety regime too.
You drop a simple glove off directing and 30min later, boom. You've got a gnome commission setting up barriers around it and paperwork for weeks to come..
A more accurate analogy would be along the lines of having jets and helicopters in the world but still using cars or trains as the main form of transportation.
Fall off a horse -> dust yourself off and get back on. There's a whole idiom about how falling off horses is a frustrating but ultimately minor inconvenience.
A horse costs way less than a scroll of Teleport, or seven Fly. Like how it's cheaper to drive than take a plane. You also don't take a plane when an hour long drive will do, and you don't cast Teleport to go from your room at the inn to the nearby village to clear out some goblins.