Some have been - old ones are unsealed and new ones are added all the time.

Not all sealed indictments listed on the websites are for "big time operators". They can be for smaller-time offenses, like local or regionaldrug-dealers (who may be tied to big-time crimibals/cartels), doctors who illegally prescribe controlled substances, fraudsters, embezzlers, etc., etc., etc.

If they were sealed, bigger-name unseals may have been Clinesmith, Sussman, Epstein and/or Maxwell.

Indictments are sealed for various reasons, for example, to prevent flight/escape (indictees may or may not know about the indictments), to protect witnesses or to buy time while investigating other people complicit in, or involved with, the crime (especially relevant in RICO or conspiracy situations).

Nice overview here: https://legalbeagle.com/6790439-sealed-indictment.html

Also, IIRC, indictments stop the clock on statutes of limitations, which allows additional time for more thorough investigation and expansion rather than rushing to court before the clock runs out with a weak case that is insufficiently vetted and formed or that is targeted against a lower-level soldier or two rather than "the head of the operation".