Because the process was set up by Boehner and approved by the Republicans. It was used with Benghazi. They will win the vote in House guaranteed..
Do you want Jim Jordan or whatever damn monkey on the Committee to grand stand politically like they did with the Mueller investigation? In the closed door session no one is grandstanding. Just a series of clowns like Gaetz, Gohmert, and others. Likely Rs on the commmittees are asking legit fact finding questions but no cameras so no obligation to perform for the audience. The hope is the Rs on the committee tell everyone that impeachment is going to happen and the President is dead in the water.Every single witness thus far is backing up the whistleblower 100% and each witness is making things worse and worse for the President based on what we've seen from public statements, etc.
The best strategy Dems have: Keep doing these private depos in the House. Lock the case down totally. No wiggle room. Then turn it over to the Senate on a platter. Let the best witnesses take the stand in the Senate trial if the Republicans want to stand by their man. Their testimony is going to be much more damaging and effective when the public hears it for the first time IMHO.
It is maximum internal political pressure on the R's, limits grandstanding, and when the Senate votes all witnesses will be heard.