With a circuit switched network, the two endpoints are connected with a dedicated circuit, meaning that for the duration of the connection, no other endpoints can use the circuit.
With frame relay, the links between the frame switches are not dedicated to any one connection, but can be used by multiple connections that may need to send traffic between two switches. The circuit is virtual (the V in PVC and SVC stands for virtual) because it looks like a circuit to the endpoints, but the physical path can be shared among many endpoints.
Contrast that with a true circuit-switched network, like standard POTS, where a circuit is set up directly from one endpoint to the other endpoint, and the entire physical path is dedicated to the two endpoints for the duration of the connection. The POTS fast busy signal tells you that all the circuits are busy, so your call cannot be completed, but that doesn't happen with virtual circuits like frame relay because the virtual circuits can use links that other virtual circuits are using.
A link between two frame switches can carry traffic for multiple virtual circuits, but you would need as many links between the switches are there are connections for real circuit switching, otherwise you could end up like POTS, where all the circuits could be busy, and you could not get a connection.