Because it is? Criminal suits are also lawsuits. The indictment is the first step in that process. Even in cases where there is no clear individual plantiff, the governing body (city, state, or nation) fills that role.
All cases are brought before and served by a judge, and in this case there would be no jury, just a review, on legal precident, whether the charges put forth by the indictment are accurate or relevant to the legal code's stated therein.
Essentially, this is a lawsuit, just one based on precedent, which really is vibe-based and heavily depends on the judge assigned to the case. When there is no precedent or the legal justification is vague, a jury is also assigned.
To be clear, an indictment is a document that kicks of the lawsuit process: "I acuse you of breaking X laws because I can present Y evidence to a judge proving you did that. Therefore, I sue you. Signed, law firm."