Why can't I override this method of my parent class?

I see this code below.

I expect the monkey instance should use the child sayName method, but actually, not like the normal, this instance uses the parent Animal.

Can some one explain why?

class Animal {
  sayName = () => {
    console.log("parent animal")
  }
}

class Monkey extends Animal {
  sayName() {
    console.log("monkey, child, extend parent")
  }
}

const monkey = new Monkey();
monkey.sayName()

This is not a wrong code! The first one is arrow function and second one is not one arrow function.

I only want to know why in this way, the instance uses the parent method rather than the normal the extends one.