You're missing the entire point of how the whole system works. The Police do not decide innocence or guilt - the courts do, and every accused person has the right to present a defence and be judged by a jury of their peers. The biggest point you are missing is that this process has to start somewhere and that is with the Police investigation and the decision to lay charges.
The Police must have evidence to lay charges and that simply does not happen if there is none. You are saying that "sufficient evidence" should be defined by law, but that is impossible to do because every single case is different, so you simply can't define what is sufficient and what isn't. The Police will lay charges if they believe that the evidence is strong enough to secure a conviction and whether or not there is any definition of "sufficient evidence" in law, it would still be the Police who would have to make that determination.
There will never be any such definition and nor should there be - unless you want to wake up one morning living in a dictatorship instead of a democracy. If you did have such a definition, then countless innocent people would be charged regardless of the fact that the Police believe there's not enough evidence to get a conviction - and counless guilty people would never be charged despite the Police believing there is more than enough evidence to prove their guilt. All because you are no longer deciding to lay charges based on the facts of the individual case, but instead on a definition that can't possibly be applied to every different situation.
The very existence of such an approach would undermine the entire judicial system.
