The short answer is because time also costs money, whether it's your own or your employees -- both for setup and ongoing maintenance. Price is an important, but not the only part of the equation here.
Sounds like an opportunity for a person with a domain expertise could make a good living doing this for companies.
We had, at lockheed, hired a contracting company that would receive all our corp cell bills, go through them and go after the carriers for errors and savings and they just kept a percentage of the savings that we made from their efforts... it worked out well for both parties.
We had, at lockheed, hired a contracting company that would receive all our corp cell bills, go through them and go after the carriers for errors and savings and they just kept a percentage of the savings that we made from their efforts... it worked out well for both parties.