Why do cats keep having kittens if they don't get fixed?

cats follow their instincts alone when they mate. when a female is in heat, she has to have sex. she doesn't think "oh, i've had enough kittens," all she thinks is "i'm in heat, i need to perpetuate the species!" all animals are like this, they act on instinct alone when it comes to mating. and their instincts tell them to keep their species thriving.