Ignoring whistleblower protections, at least the whistleblower in some respects is personally involved, even if very indirectly.
Trump denies that Biden had anything to do with the hold up of Ukraine aid
Biden played no role in WH negotiations with Ukraine
If you want to investigate Hunter Biden separately, be my guest. But what does that have to do with what Trump did or did not do? If Trump denies that H Biden had anything to do with the Ukraine aid, how does putting him on the witness stand benefit Trump and the GOP at all in this impeachment trial?