The average time for the best horses in the US, if you take the time of the triple crown is the same as 50 years ago.
However humans speed across distance has improved by a lot. What could it be?
1. Training has changed drastically in humans but was already peaked in horses 50 years ago
2. Same but for diet
3. Doping is much better for humans now than 50 years ago. Same for horses.
4. Doping is the same but antidoping is much worse (while consistent for horses)
5. Talent pool is much bigger in humans while horses stayed the same or declined; made the chance of improvement much likely.
6. Peak genetics for horses was achieved and can’t be much improved and is the main factor for horse speed. Humans genetics does not play as big a role.
7. Technology improvement benefitted humans much more than horses. I guess it’s basically the same track for horses.
8. Humans are driven by breaking records while a horse does not care much about beating a world record or seeing another one do it.