> Since there is the concept of sealed indictment, why are all the indictments not sealed (or none of them)?

There are a variety of reasons to seal an indictment (which will only remain sealed until the defendant is arrested), one of which os to facilitate arrest and avoid flight, another is to protect information in the indictment for as long as possible, especially if there is information that may be sensitive to ongoing investigations beside that of the person indicted, or if there is concern for the safety of victims/witnesses identified in the indictment.

> I guess this is to preserve the person that is accused

No, it is not. It is mostly to protect against actions of that person or their confederates, not to protect them.

Most indictments are never sealed (if the accused has been arrested on other charges, for example, there isn't much use in trying to keep it secret from the accused), and the ones that are always become unsealed in order to go to trial. There are a number of reasons to file an indictment under seal - ranging from juvenile records to managing flight risks of the accused.

The one that is probably most pertinent in this case has to do with testimony in front of a grand jury: both encouraging it to occur, and preventing it from being tampered with by the accused.

> I am from France and have no stakes nor interest in the political discussion

The impact of U.S. politics goes well beyond its borders.