The whole idea of "we have to hear from the witnesses" ignores the way our legal system actually works. Fact witnesses are supposed to testify to relevant facts -- not give legal conclusions or opinions -- that are in dispute. It is not at all uncommon in the law for a case to be dismissed without hearing from all the witnesses simply because the undisputed facts -- and sometimes even the allegations themselves -- do not support the charge.
The second Article of Impeachment claims that Trump "Obstructed Congress" by claiming executive privilege with respect to some witnesses and documents. Well, he did claim executive privilege, so the facts aren't in dispute. So why are witnesses required? In that case, the only question is whether the allegations themselves even state something that constitutes a "high crime or misdemeanor", and that's a legal conclusion for the Senate to reach, not something for which testimony is required. Fact witnesses have nothing of relevance to offer.
The first Article claims obstruction based on Trump's phone call. But we already have the text of that phone call, so that's not in dispute. In truth, this isn't a factual dispute at all. It's whether you believe that the actions of which Trump is accused actually constitute a high crime or misdemeanor. Even if Trump did direct that the aid be withheld for a few weeks, and even if his motive was to pressure the Ukrainians to turn over anything they had on Biden, it is entirely possible to conclude that is not an impeachable offense.
The call for witnesses isn't about trying to resolve factual disputes. It is about trying to air the allegations again so as to try to influence public opinion.