[@Revans Exile]

The only reason jockeys aren't needed in dog racing is because they run a "rabbit" along the edge of the course that the dogs hone in on and chase. They're trained to pursue the rabbit, now just a glorified dog toy on a rail.

The jockey performs the same role, except he's responsible for not only keeping the horse on track but to control its placement in relation to other horses and their pacing. The Equestrian sport is really a team sport between the human rider and the horse. If you listened to any professional analysis then they'll focus on the human as much as the horse since the rider's intuition or knowledge of horse racing (as a veteran or otherwise) can help win the race by know when to tell the horse to back off or let up.

And given their social herd animals if the one at the head - or any of them - sees shiny and they go off to investigate the squirrel the others may as well follow. They'd also probably pace themselves to the lead horse without "encouragement".

Horses are by their nature a herd animal and if you let them all out on the track without a rider to give them direction then you'll be seeing a lot of nothing resembling a race. Most likely socializing or ambling about on the tracks. Or if they're an aggressive horse by nature they'll probably start biting each other. Jockey needs to control that.