More like it's not wise to to put somone under oath who will only further incriminate the president.They can, if the Republicans in charge allow them to.What makes you think that they can't call witnesses and subpoena documents that were obstructed from reaching the House?They've already heard the best the prosecution has, so the only new evidence would be from the defense. Is that what you want, to see the charges be reduced to shreds and lying in the dust?
Why wouldn't they allow testimony that explains how "perfect" the president acted?
Because they know that putting someone under oath in front of the howling mob isn't a wise thing when they don't have to.
If there were excuplpatory testimony to be had by administration officials, it would have happened already.
That's certainly one belief system. Since none of us are privy to the president's inner thoughts, it's only speculation.