You really aren't.
There are lots more clauses than just HOY.
In some cases:
Win a grade one(assuming the horse hasn't won on yet)- one mill
every additional grade one- 1 mill
wins championship- 1-3 mill
wins HOY- 1-5 mill

Thats how these deals are done.
Bonus clauses became popular because farms wanted to gamble on a prospect who had upside. Owners didn't wanna lose all that possible upside and would wait until the horse was done racing to do a deal.
On these clauses, they way they are designed, believe me the last words spoken by the farm that buys the rights are "Man, I sure hope I get to send you another 5 million". Seller "me too!!"