You want to know why?
Because..........Commissions are negotiable.
Period, end of story!
(btw - if you meet with any agent who tells you that "all the other compaies are charging X%" , they are breaking a serious federal anti- trust law by trying to set a commission)
The real answer is that costs across the board are pretty much the same for all brokers and agents, its only a matter of what services are provided for the money being spent. Some companies will charge less for less in service. Some offer premium services whether you need it or not.
If you wish to know what services the commission you are paying is getting you, then make a list and interview some agents. Most agents know that sellers seldom actually interview more than two agents. Interview agents and ask them what services they provide and what commissions they expect to receive for that level of service. Any agent is free to tell you they will not work for less than $"x" and that is their right. What is prohibited is negotiating commissions with the other brokers to keep rates higher. Costs are high, prices are low, sales volumes are also low, and agents do have the right to earn a living.