Because an investigation where the suspect is permitted to sit in during interviews with witnesses is a bad investigation. Employers, universities, law enforcement federal agencies--none of them permit suspects to participate in the investigatory process with the exception of giving their own interview and providing their own documents. Why? Because it's stupid to permit a suspect to hear what other witnesses have to say.
It's only once the investigation proceeds to an adjudicatory phase that the suspect is permitted to hear the evidence against them and cross examine witnesses. We aren't there yet. Republicans know this. Most of the right wing media know this. But the facts coming out thus far in the investigatory phase are so bad that Republicans and right wing media are misleadingly attacking the investigation as unfair despite it following the same process as virtually every other type of investigation that is done.
It would be stupid for the Democrats to change the fundamental nature of an investigation, thereby undermining it, because Republicans and right wing media are lying, hypocritical pieces of shit.
But I'm sure you knew that and you were only asking question.