It's more that once they have the experience they become more difficult to handle and keep their mind on their racing job. That's why when you hear of a subfertile stallions coming back to racing, like George Washington or Starspangledbanner, they have a few months of adjustment.
I also had gotten the impression that breeding and racing require a different type of "fit" and musculature that I'm not sure a breeding stallion could really race competitively.
I know that if the horse was really "great" that it would be fun to keep them racing but the $$ is in the shed as well as maybe having one of the rare stallions that can pass on some of their ability. Tapit comes to mind and maybe Uncle Mo (I think it is although he's still young in his breeding career).
No one can make you feel inferior without your consent. - Eleanor Roosevelt
Minnie is correct. Insurance rates are based on value and value is based (for a breeding animal) in part on what he's worth to breeders.
This is why we see so many very successful colts head to the shed rather than race on. Faced with (literally) insurance premiums that could only be covered by winning the Dubai World Cup and the Breeders Cup, most owners take the chance to sell a colt to a partnership for breeding.
Resident racing historian ~~~ Re-riders Clique
Founder of the Mighty Thoroughbred Clique