People who are trained in interpreting and delivering a script usually give a better performance. Performance matters to listeners. Other authors are great interpreters of their own work. Bringing a book alive in a recording booth is different from amateur theater, reading to children and friends, or talking on the telephone. Some skills are transferable, and some aren't.
Recording sessions can have many stops and starts, even with highly paid professional narrators.
Inexperienced narrators find these interruptions unnerving. Newcomers to the studio are prone to focus on their own performance, rather than concentrate on connecting with a listener. That's natural, but anxiety comes through in anyone's voice. Likewise, fatigue, a cold induced hoarseness or nasality, coffee nerves, or other stresses will be recorded on tape. Many authors improve quickly with rehearsal and voice coaching.