Why most LTE operators manage voice calls via CS-fallback function?

Traditionally, voice services have been provided over circuit switched (CS) networks in wireless systems.
But LTE is a purely packet switched (PS) system and, thus, requires the deployment of IMS in the operator’s Core Network to integrate voice services and to ensure Quality of Service and adequate charging mechanisms.
However, most commercial systems today do not have IMS integrated in the Core Network. This prevents the deployment of voice services over LTE (VoLTE).

Typical LTE networks are designed for downloading, streaming and web browsing – basically handling data traffic. These networks are not sensitive to coverage, capacity and mobility issues that can be faced during voice and video sessions.

As a result, most LTE operators manage voice calls via CS-fallback function by directing session to 2G or 3G networks. However In such cases, mobile users are deprived of high definition voice and video experience, which can only be delivered through an all-IP network.