Why has not a single person called to testify said they saw or heard him say or do anything illegal.?
You assumptions are incorrect, the act of withholding the funds was illegal. Trump is the person that directed everything to be done illegally, he then released the call of him asking for the announcement of the investigation, which was illegal, Trump's own words.
Mulvaney then comes out and confirms it to the country from the podium of the United States of American speaking for the President.
Then the President himself in front of reporters again said that if everyone was being honest he would want Ukraine to investigate the Biden's.
We all saw that one, from his own mouth.
And the rest of the people have been commanded by the President of the United States of America to illegally dismiss the legal subpoena from congress, so also he stonewalled and obstructed justice.