Because 'reasonable pricing' probably doesn't cover the cost of ensuring that on arrival and subsequent departure the GA aircraft and its occupants are segregated from the critical part of the security restricted area. At airports with restricted space which handle commercial traffic required to be protected in accordance with a National Aviation Security Programme, it takes either considerable staffing dedicated to managing GA crew and passengers, or a significant investment in security technology, with appropriate personnel ready to respond in the event of the GA occupants straying, to ensure that the Critical Part is not infringed. Infringements of the CP by unscreened people are significant security events, and can seriously disrupt airport operations. It is much simpler for airports to devolve GA handling to agents who can provide demarcated areas (1). That transfers the onus onto the agent to satisfy the airport's security requirements, and permits the airport to say to its own security inspectors, in the event of a breach, "it was the agent that let us down, and we're taking appropriate steps to resolve the issue with them", and frees the airport of the cost of providing an area segregated for GA and staffing its security.
Eventually, it all becomes too much, and only top end GA can absorb the cost of segregation. Its all about the economics of operating an airport, really
HB
(1) 'demarcated area' means an area that is separated by means of access control either from security restricted areas, or, if the demarcated area itself is a security restricted area, from other security restricted areas of an airport;