Even that increases the risk of a leak. If DOJ is working on 100 cases, as one of yesterday's Q-drops suggests, there is no advantage to "sharing" them with a court, until close to the time an arrest is intended to be effected.
The reason to have paperwork in the court is to protect the fairly tight timelines that due process provides between arrest and being confronted with charges.
Another thought occurred to me. Assuming the charges are in the nature of material support for terrorists, a general approach in open court would be to hide the ID of the terrorist, in order to protect an ongoing investigation or military operation. That precaution would not be required if the terrorist is dealt with first. Maybe a substantial part of the delay is dealing with the outside actors, so they can be revealed when the inside actors are arrested and charged.
Mark