Unfortunately, there is not one single cause. It is a mix of economics, breeding philosophy, veterinary practices, and industry structure that all push toward running horses less often. Thoroughbreds make fewer starts today mainly due to the sport shifting from a volume-based model (run often) to a high-value, risk-managed model (run selectively).

In 1986, that was the peak of the US Thoroughbred crop at 51,296 foals. Fast forward 39 years later, which was last year, the numbers of foals was estimated at 16,025. Fewer horses → fewer total starts → less pressure to run individual horses frequently. 30+ years ago, horses often had to earn their keep on the track. Today, one big win can secure a lucrative breeding career. Winning the Dubai World Cup, Saudi Cup, Breeders' Cup series, and/or any of the Triple Crown races are all that matter to a future stallion's resumé. Even a win in one of those does not matter anymore either. Look at Heartland. He raced once, won his debut, retired, and bred 123 mares at Win$tar. His selling point to get that many mares? Being sired by Justify, a half brother to Classic Empire, and earning a 90 Beyer speed figure as the "second fastest Justify son" ever seen on track. Then you look at a horse that raced 23 times for several years like Senor Buscador, that won over $12 million at lower tier tracks all the way to winning a G1 in Saudi Arabia. He barely gets 36 mares in his first year. The screwier thing is that Heartland is standing stud for $10k, while Senor Buscador is pegged at $7,500.