That's because underinsured motorists coverage (and uninsured motorists coverage) is an adversarial coverage between an insured and his own insurance company. The plaintiff's own insurance company stands in for the other driver's insurance shortfall and only pays if the other driver is negligent.
The UIM insuring agreement:
"We will pay compensatory damages which an insured is legally entitled to recover from the or operator of an underinsured motor vehicle because of bodily injury sustained by an insured and caused by an accident."
If the other driver is not negligent then the insured is not legally entitled to recover from the other driver. The insurance company has good reason to defend the other driver.