This occupation is selfless. You are putting your life at risk against a force stronger than you, knowingly, to save other people that you don't know.
Being a police officer isn't the same level of risk/selflessness/helplessness. Cops have guns, and the entire American legal system on their side against anything that threatens their livelihood. You can't stop a fire from burning your face off as simply as you can arrest and lock up/shoot a criminal for assaulting you. This makes police work a more "safe/protected" position in terms of being able to self defend against workplace hazards.
You don't see many female firefighters because they lack the moral and physical fortitude to do the job. It takes guts, strength and selflessness. They won't even be trained and subsequently PAID to rescue someone. I feel like I know many men that would do this if they had the chance, just to be a hero and save someone's life. I don't know one woman that would.