onChange triggers on every keystroke

I have a onChange event attribute in an input box in a component,

<input
    type="text"
    required
    className="form-control"
    onChange={this.onChangeUsername}
/>

which is handled by this onChangeUsername() function

onChangeUsername(e) {
    console.log(e.target.value);
    this.setState({
      username: e.target.value,
    });
}

this function is called on every keystroke. Why is that?