Why is the button not pressed to the bottom of the container?

Question

Android

Why is the button not pressed to the bottom of the container?

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:id="@+id/container1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <LinearLayout
            android:id="@+id/container2"
            android:layout_width="wrap_content"
            android:layout_height="60dp"
            android:layout_alignParentBottom="true"
            android:layout_centerHorizontal="true"
            android:orientation="horizontal">

            <Button
                android:id="@+id/button18"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="Button" />
        </LinearLayout>
    </LinearLayout>
</RelativeLayout>