Why would you call a million witnesses in to testify in a hearing where the accuser has not made a complaint under oath, won't turn over evidence, all the witnesses contradict/don't validate her version, and there is a solid chance she won't show up because her story is incredibly flimsy and will be easily picked apart under scrutiny??
Because you're vetting a candidate for a lifelong appointment to one of the 3-4 highest, most powerful offices in the country. If Ford's story falls apart, can't be corroborated, seems insincere, that's a point in his favor, not against it. And he needs them. In a criminal process, the defendant is not guilty by default, and there needs to be a compelling case to change that. In vetting a candidate for high office, it's the reverse - they're unfit by default and must show enough to change that. There's a lot of rhetorical effort on convincing people it's the opposite, that they're entitled by default and something has to "disqualify" or "keep them out", but ultimately the focus is on the office, and the candidate's gotta prove they're suitable for it.
That requires a lot of things, but one of them is that they need to clear up, as much as possible, any serious allegations of misconduct. That means refuting the allegations if possible, but if not the kinds of things you're expecting would suffice pretty well too. Either way it means as thorough an investigation as possible. Blowing off the allegation, deciding in advance it must be all mades up by the badpeoples, or doing a cursory sham investigation just makes the candidate unvetted and the process illegitimate.