Why can't you name and additional insured on Workers Compensation?

Answer 1

Because this form is designed for general liability policies because the insurer is obligated to defend and indemnify that additional insured in accordance with the policy terms and conditions.

Generally, the insurer remains obligated to fulfill its obligations under the policy even if the additional insured's negligence is the sole cause of the liability for which damages are being sought. Insurers, however, always try to limit their exposure as to additional insureds and the intent is not to cover other company's employees. Coverage for those employees should be provided by their employer and most General Liability specifically exlude coverage for employees.