Gazebo Launches Empty

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I've been following the URDF/XACRO tutorials and I've run into an issue. When I launch Gazebo the model never shows up.

• Ubuntu 18.04
• ROS Melodic

My gazebo.launch file:

<launch>

    <!-- This will load Basic_Geometry_Pig as the URDF model -->
    <arg name="model" default="$(find little_pig_atv)/basic_geometry_pig.urdf.xacro"/>

    <!-- Setting some default args to start Gazebo with -->
    <arg name="paused" default="false"/>
    <arg name="use_sim_time" default="true"/>
    <arg name="gui" default="true"/>
    <arg name="headless" default="false"/>
    <arg name="debug" default="false"/>

    <!-- Load an empty Gazebo world with the args above -->
    <include file="$(find gazebo_ros)/launch/empty_world.launch">
        <arg name="debug" value="$(arg debug)" />
        <arg name="gui" value="$(arg gui)" />
        <arg name="paused" value="$(arg paused)"/>
        <arg name="use_sim_time" value="$(arg use_sim_time)"/>
        <arg name="headless" value="$(arg headless)"/>
    </include>

    <!-- This is set with code from line 4 -->
    <param name="robot_description" command="$(find xacro)/xacro.py $(arg model)" />

    <!-- push robot_description to factory and spawn robot in gazebo -->
    <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" args="-z 1.0 -unpause -urdf -model robot -param robot_description" respawn="false" output="screen" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher">
        <param name="publish_frequency" type="double" value="30.0" />
    </node>

</launch>

My basic_geometry_pig.urdf.xacro file:

<?xml version="1.0"?>

<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="basic_geometry_pig">

    <!-- Some color definitions for the URDF Robot -->
    <material name="blue">
        <color rgba="0 0 0.8 1"/>
    </material>
    <material name="black">
        <color rgba="0 0 0 1"/>
    </material>

    <!-- Basic Geometry of Body -->
    <xacro:property name="width" value="0.680618" />
    <xacro:property name="height" value="1.065022" />
    <xacro:property name="length" value="1.91" />

    <!-- The Ackerman Steering Hub -->
    <xacro:property name="ackerman_joint_diameter" value="0.09" />
    <xacro:property name="ackerman_joint_width" value="0.09" />


    <!-- Wheel Properties -->
    <xacro:property name="front_wheel_width" value=".254" />
    <xacro:property name="front_wheel_diameter" value="0.5588" />
    <xacro:property name="rear_wheel_width" value="0.2794" />
    <xacro:property name="rear_wheel_diameter" value="0.6096" />

    <link name="base_link">
        <inertial>
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
            <mass value="49.8952"/>
            <inertia ixx="11.48" ixy="10.52" ixz="1.737" iyy="1.737" iyz="11.48" izz="10.52"/>
        </inertial>
        <visual>
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
            <geometry>
                <box size="${length} ${width} ${height}"/>
            </geometry>
            <material name="blue"/>
        </visual>

        <gazebo reference="base_link">
            <material>Gazebo/Black</material>
        </gazebo>
    </link>

    <!-- Ackerman Steering Macro -->
    <xacro:macro name="ackerman_steering" params="suffix reflect">
        <link name="${suffix}_ackerman_component">
            <inertial>
                <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
                <mass value="0 ...