Updates to variable not visible throughout the script !!!

Came across this problem while answering this question

https://www.experts-exchange.com/questions/20671155/Simple-question-lotsa-points.html

#!/bin/bash

i=0
sum=0
avg=0

ls -l | awk ' { print $5 } ' | while read line
do
    if [ $i -eq 0 ]
    then
         echo -n
          i=1
    else
          sum=`expr $sum + $line`
          avg=`expr $sum / $i`
          i=`expr $i + 1`
    fi
echo $avg
done

echo $avg


This script calculates the average file size in a given directory
In this script, echo $avg inside the while loop prints expected values whereas outside the loop it prints 0 or whatever value I initialize it to before the loop

Can anyone explain why is this so ?