GSA gets a cut of contract purchases that go through them but this makes sense because it incentivized them to be better alternatives than an agency’s own contract shops. Typically working through a GSA schedule like this can lead to faster turnaround and lower prices - nobody can charge the government more than their best commercial customer, legally, under this framework. If you don’t get value, put your own RFP on the street.
The incentive to get more dollars if you get more under management as a contract-service-provider does make sense. They’re not a monopoly on procurement in government, despite the author writing a book about monopolies.
Note: in some cases they have monopolies on some things in government, but not procurement.