By paying marketing fees upfront, consumers assume more risk. When customers ask for a commission reduction, most agents are unable to comply, because they can only speak for their own fees. If you are selling a home and ask an agent to lower the commission, the agent knows that if fees are lowered across the board, there will less enthusiasm all around for the home. The other agents have other homes to show in which they will receive their full fee. Ironically, it is the sellers most in need of selling who typically make such requests.
Commissions are negotiable by law, however.
Realtors are one of the few professions that are paid on contingency and often without a contract. This is risk-reward entrepreneurism, and that takes a special kind of fortitude. Imagine going to work each day not knowing when and if you are going to be paid. Imagine having to put a smile on after learning that the customer you have assisted for weeks, looking at home after home, has just put a contract on a home listed by another agent. Imagine working with a seller who prices the home too high against your advice and then fires you when the home doesn't sell.