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.
Nevertheless, most current commercial systems do not have IMS integrated in their CN. This stops the deployment of voice services over LTE.

Standard LTE networks are built for downloading, streaming, and web browsing ; and generally managing data traffic. These networks are not sensitive to capacity, coverage, and mobility problems that can exist during voice and video sessions.

That is why most LTE operators handle voice calls via CS-fallback function by directing sessions to 2G or 3G. 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.