Would that not give them the witnesses and documents they need to conduct meaningful oversight?
No, DUmmie, it would not. Articles of impeachment do not just magically transmogrify everyone into the House into some sort of super-sleuth who gets warrants and subpoenas for everything. They can subpoena me all day every day, and I will simply tell them to perform an anatomically impossible act, and there won't be a damned thing that they can do about it. They can try to send the Sergeant-at-Arms after me and I have the legal right to blow his head clean off if he tries to take me anywhere. I am not subject to the House's whims, nor are almost everyone else who is not directly employed in the House and a scant few offices of the Executive Branch, none of whom matter here. They can try to subpoena Donald Trump, Jr., and he has every legal right to simply extend a middle finger to Pelosi in response.
Here's what would happen if the House listened to kentuck:
Pelosi: "We have passed articles of impeachment against Donald Trump. We have subpoenaed Donald Trump, Jr. to demand answers about the Trump Tower meeting in 2016."
DJTJ: "Go piss up a rope."
Pelosi:
*hyperventilates a lot, and is ultimately forced to admit that she has absolutely no authority to demand or compel DJTJ's presence*
*Don Lemon's head simultaneously explodes on camera at the same time as Jim Acosta's*