The Dems did the investigation. They closed it out when they voted articles of impeachment. They gave up on getting more testimony, but they could have waited for the federal courts to decide if McGahn could be compelled to testify under oath. If they had waited, and if the courts had decided the White House counsel could be compelled to testify, the Dems could have had his testimony.
But it was waaaay too urgent they end the investigation toot sweet and pass them articles of impeachment. Nancy was worried the voters in 2020 would decide the matter if she didn't hurry it up!
The investigators/prosecutors (the Dems in the House) completed their work. Now it goes to the Senate to act as jury. In any legal proceeding the prosecution does not get to add in more charges under the original indictment, and they don't get to bring in extraneous new testimony. That is, if someone is accused of arson for burning down a building, the prosecution doesn't get to bring in witnesses to claim the defendant also committed bank robbery. The prosecution doesn't get to try to convince the jury to convict the defendant of that bank robbery.
The prosecution can do a new investigation into the bank robbery and they can bring new separate charges of bank robbery, and then they can present that new case in a different trial.
The House could do more investigations and pass more articles of impeachment. But they don't get to now add in new charges to the existing articles, and they don't get to hold up the trial saying they want to do more investigation into another new additional potential future charge against Trump.
Furthermore, the jury in a trial doesn't bring forth new additional evidence. The jury doesn't add in new witnesses. The prosecutor is only allowed to bring forth evidence specific to the charges. That evidence is contained within the Articles of Impeachment they passed. The jury gets to hear that evidence and then decide if it proves the case.
All the Senate is obligated to do is review the evidence already developed, which is all public domain at this point, and then render their decision.