Because fire-fighting is fairly straightforward in terms of skills needed and everyone agrees such fires are a bad thing.
Policing is intensely social and political, requires threat identification, deescalation, due process, knowledge of rights and legal intricacies, etc. It is full of subjective dimensions that are impacted by individual personalities, prejudices, training, moods, fears, etc.
While few people actively try to subvert or aim to repurpose volunteer fire fighting to act against community interests, volunteer policing would be a very different story. There's a lot of money to be made in crime and a lot of power to be had on either side of the equation that can be gained through force or intimidation.
Not to mention the kind of people who are attracted to vigilantism are not always the kind of people you would like to hold such power. No matter how bad this is with the police, at least they have some barriers to entry and weed out some of the worst.