Because this is an investigation. Like Mueller held investigations in secret. Witnesses cannot co-ordinate testimony.
Has the DOJ followed up the criminal referrals about this Ukraine scandal, the House would not have to do it themselves.
Before the previous impeachments, a Special Prosecutor was named (Cox, Starr, Mueller) held grand jury investigations and produced a report submitted to congress. The William Barr DOJ denied any investigation, a cover up.
Articles of Impeachment will be drafted in public hearings. This is the procedure laid out in the constitution.
Arguing process shows a lack of defense of the allegations. Why aren't Trumpians shouting about the allegations?