In the ConstraintLayout below, why wouldn't button expand to fill the width of parent?

    <androidx.constraintlayout.widget.ConstrantLayout
        ...>
        <Button
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:text="Button"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"/>
    </androidx.constraintlayout.widget.ConstrantLayout>