URDF - wheels joints are stuck to base_link

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After spending so many hours figuring things out, I finally got to make a two wheeled robot that gets controlled with diff_drive_controller using ROS. However, whenever i publish cmd_vel like the following:

rostopic pub /diff_drive_controller/cmd_vel geometry_msgs/Twist -- '[1.0, 0, 0]' '[0.0, 0.0, 1.2]'

the robot moves in a weird way: the base_link seems to be fixed to the wheels. Whenever the wheels move, base_link moves with the wheels. What I want is the base_link to stay still and the wheels to rotate.

Here is my urdf of the two wheeled robot:

<robot name="asst1"> <gazebo> <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so"> </plugin> </gazebo>

<link name="base_link">
    <collision name='collision'>
        <origin xyz="0 0 0.1" rpy="0 0 0" />
        <geometry>
            <box size=".3 .3 .05"/>
        </geometry>
    </collision>

   <visual name='visual'>
       <origin xyz="0 0 0.1" rpy="0 0 0" />
       <geometry>
           <box size=".3 .3 .05"/>
       </geometry>
   </visual>

   <inertial>
       <origin xyz="-0.1 0 0.1" rpy="0 0 0" />
       <mass value="5"/>
       <inertia
           ixx="0.03854" ixy="0.0" ixz="0.0"
           iyy="0.075" iyz="0.0"
           izz="0.03854"/>
    </inertial>
</link>


<link name="left_wheel">
    <collision name="collision">
        <origin xyz="0 0 0" rpy="0 1.5708 1.5708" />
        <geometry>
            <cylinder length="0.05" radius="0.1"/>
        </geometry>
    </collision>

    <visual name="visual">
        <origin xyz="0 0 0" rpy="0 1.5708 1.5708" />
        <geometry>
            <cylinder length="0.05" radius="0.1"/>
        </geometry>
    </visual>

    <inertial>
        <mass value="1"/>
        <inertia
            ixx="0.002708" ixy="0.0" ixz="0.0"
            iyy="0.002708" iyz="0.0"
            izz="0.005"/>
    </inertial>
</link>


<link name="right_wheel">
    <collision name="collision">
        <origin xyz="0 0 0" rpy="0 1.5708 1.5708" />
        <geometry>
            <cylinder length="0.05" radius="0.1"/>
        </geometry>
    </collision>

    <visual name="visual">
        <origin xyz="0 0 0" rpy="0 1.5708 1.5708" />
        <geometry>
            <cylinder length="0.05" radius="0.1"/>
        </geometry>
    </visual>

    <inertial>
        <mass value="1"/>
        <inertia
            ixx="0.002708" ixy="0.0" ixz="0.0"
            iyy="0.002708" iyz="0.0"
            izz="0.005"/>
        </inertial>
</link>

<gazebo reference="left_wheel">
    <mu1 value="10000.0" />
    <mu2 value="10000.0" />
    <fdir1>0 1 0</fdir1>
    <minDepth>0.01</minDepth>
</gazebo>


<gazebo reference="right_wheel">
    <mu1 value="10000.0" />
    <mu2 value="10000.0" />
    <fdir1>0 1 0</fdir1>
    <minDepth>0.01</minDepth>
</gazebo>

<joint type="continuous" name="left_joint">
    <child link="left_wheel">left_wheel</child>
    <parent link="base_link">base_link</parent>
    <axis xyz="0 1 0"/>
    <origin xyz="0 -0.175 0.1" rpy="0 0 0"/>
    <!--<limit effort="100" velocity="10.0"/> --> 
</joint>


<joint type="continuous" name="right_joint">
    <child link="right_wheel">right_wheel</child>
    <parent link="base_link">base_link</parent>
    <axis xyz="0 1 0"/>
    <origin xyz="0 0.175 0.1" rpy="0 0 0"/>
    <!--<limit effort="100" velocity="10.0"/> --> 
</joint>


<transmission name="right_tran">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="right_joint">
        <hardwareInterface>VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="right_motor"/>
    <mechanicalReduction>1</mechanicalReduction>
    <motorTorqueConstant>1</motorTorqueConstant>
</transmission>

<transmission name="left_tran">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="left_joint">
        <hardwareInterface>VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="left_motor ...