why don't short haired animals grow long hair?

Because animals have genes that control their appearance, everyone does. Some animals have the gene for long hair, whereas others have the gene for short hair that will stop growing at a certain length, or slow the growth process down.

Same reasons that some people can't grow their hair long and others have very long hair.

My daughters are twins, one is dark haired and it is very thick and long. Other daughter hair is thin and when she tried to grow it long it would break off.

Dogs are the same. If parents genes say short then fur is short. Other genes determine long hair.

The animals have genes that give it it's short hair. I you want an animal with longer hair my best guess is you get an animal with long hair or breed your current pet (short haired) with another pet (long haired) and hope to get an animal with longer hair. hope this helps any!

Because somewhere in their genetic make up it tells the hair to be short. It is the same as like hair color, texture, etc.

My best guess would be genetics?