Discovery is terribly intrusive and invasive, more than you would ever think.

It's one of the huge reasons that lawsuits are often settled, because ain't nobody want all the dirty laundry aired.

Discovery asking for a peon's diary is likely to get squashed pretty easily, discovery asking for the president's personal diary is likely to be approved if there's an argument it applies (and likely you'll see what happened here, it's provided sealed, and if both sides agree it's applicable, it's unsealed).