Rationale
The basic rationale for the exception is that employees are under a duty to be accurate in observing, reporting, and recording business facts. The underlying belief is that special reliability is provided by the regularity with which the records are made and kept, as well as the incentive of employees to keep accurate records (under threat of termination or other penalty). The exception functions to allow the record to substitute for the in-court testimony of the employees, but it can only substitute for what the employee could testify about. The availability of the declarant (the employee whose testimony is being replaced by the record) is immaterial for the purposes of this exception.
Read more about this topic: Business Records Exception