Why are fillies/mares not spayed like other female animals?

CDog is spot on again.

In most cases, no, fillies and mares are easily managed and do not need to be spayed like ones cat or dog. Its colts and stallions that, because of handling and behavior, are often gelded (the neutering procedure for horses) if they are not used for breeding.

For fillies and mares the process is invasive, expensive, and unnecessary unless there are some serious health issues associated with a particular filly/mare's reproductive tract. I have yet to actually meet someone who's had it done.

Becuz large female animals require a sterile, surgical area for the procedure...and because spaying of any animal, human or horse results in brittle bones and fractures due to the lack of estrogen.

Besides the cost, it's easy to keep them from being bred in most cases. Stallions are usually kept well contained. Reproductive cancers aren't a huge issue in horses, either, unlike dogs. Great question.

Because it's a very invasive and costly procedure that is often more painful and expensive than it is in the long run for the mare and her owner.

no need....stallions need to be castrated because they go crazy and can be very dangerous. One of mine did this and we gelded him that very day. Once gelded, no need to sterilize a mare