Conceivably, you could run trials by having a judge (or panel of judges) bring forth the evidence they thought was important. Instead, many countries have a system where one party prevents “one side” of a case and another party presents the “other side”. How did this come about?
This is what cops are supposed to do.
They’re supposed to investigate a crime and narrow down from “everyone” to “the person who did it”.
Once they have enough evidence they bring it to the prosecutor who decides if it would win in court.
Then the state presents that evidence to a judge, and the defendant gets to defend themselves.