My take is the same as yours. Too much ego to have an expert look at the issue.
Hilary’s sloppiness and (justifiable) paranoia extended to her server set-up. She was sloppy with national security with her server setup. There are not many people she could confide in and ask questions about her illegal server setup. So Hillary and Huma winged it. She figured national security would not be a big issue because after all, she is HILLARY!!