When the winning horse crosses the finish line of the Kentucky Derby today, it’s almost certain it will finish somewhere between one minute and 59 seconds and 2:06. Every Derby winner in the last 70 years has finished in that band.

The Derby record of 1:59.4 for a mile and a quarter was set by the legendary Secretariat in 1973, and only once since then, in 2001 by Monarchos, has the winner finished under two minutes.

Meanwhile, human runners keep getting faster. The world record in the men’s 800 meters—which is around half a mile and historically takes roughly the same amount of time to complete—has fallen three seconds in the same time period. While it’s not an exact comparison—Derby times can be affected by adverse conditions or in-race tactics—the evidence suggests thoroughbreds have hit a plateau.

The stalled progress of horses is even more puzzling considering the considerable time and money spent breeding horses to get faster. Unlike humans, horses have been selectively paired for centuries to produce faster offspring. Secretariat was the foal of Bold Ruler, a champion stallion, and in his stud career, Secretariat sired 663 foals. His great-great-great grandson is American Pharoah, the winner of the 2012 Kentucky Derby. Yet despite all that careful breeding, none are as fast as Secretariat.

There’s a few theories as to why. For one, equipment for humans has improved significantly since 1973. Running shoes are better, tracks are better, training technology is better, while similar gains haven’t been made in thoroughbred racing (a horseshoe is still a horseshoe). There’s also the question of desire. Humans care about records and want to break them. Horses may want to run fast, but don’t have the same motivation to squeeze out an extra ounce of effort to gain a hundredth of a second.

But the most convincing explanation is biological. According to a 2008 study of thoroughbreds, greyhounds, and Olympians (pdf) in the Journal of Experimental Biology, there is a physical limit to just how fast any animal, humans included, can run; horses and dogs appear to have reached it.

One possible reason is the selective breeding of thoroughbreds since the 1700s—so extreme that 95% are descended from a single Arabian horse—has reduced the genetic variation within horses and created an evolutionary bottleneck. By excluding slower horses from their pedigrees, breeders may have inadvertently limited the potential of race horses. “Selective breeding starting with different equine stock could perhaps yield faster horses,” wrote Mark Denny, the paper’s author.

That goes against decades of received wisdom—so there’s little reason to expect Derby times to improve anytime soon, if ever.