Because that pattern doesn't carry over into the Spanish names for other animals. Some animals have a single name, regardless of gender; others have names that change completely depending on gender (e.g. "el caballo" is a stallion, or male horse; "la yegua" is a mare, or female horse).
The dog (male & singular) = El perro (also an insult, don't use it to call a person)
The dog (female & singular) = La perra (also an insult, don't use it to call a person)
The dogs (male & plural) = Los perros (also a group of female with at least one male)
The dogs (female & plural) = Las perras (also an insult, don't use it to call people)