1) The members of Congress were scheduled to ask questions AND there would be a 30 minute block allocated to staff attorneys designated by the Democrats and GOP. So the question was a straw man.
2) The answer to why Democrats would want a subject matter expert to ask questions is pretty obvious. It's the same reason Wray wouldn't walk into a years long investigation and take the lead on questioning a witness, versus let the career people who were handling the case take the lead. The answer is obvious. A staff lawyer is far more likely to effectively question someone on the specifics. And now you know why Barr didn't want that to happen.
Surely you've seen some hearings before. The 5 or 7 minutes per member rules are actually terrible if what we're interested in is information because no one can delve into any details before their time runs out. It's worse when the witness decides to filibuster and run out the clock, which we saw quite a bit of from Barr.
So your question is, essentially, why do Democrats want to allocate a 30 minute block to someone who has a mastery of the facts to effectively ask questions of Barr?