I've worked such a job at a large enterprise. It really does feel like firefighting (minus all the smoke inhalation and physical strain and death risk).
However, not only is not everything a true positive, probably only about 0.001% of things are true positives, among a sea of alerts and reports and dashboards across myriad systems. Some coming from your SIEM, some generated by security appliances and products, some from internal employee reports.
An ideal place will have people who continuously work on trying to reduce alert fatigue and false positive noise - but, in practice, at most big companies it's probably like working at a fire station and getting hundreds of dispatch calls per hour, every hour, every day, each about a potential fire at a different residence. And then you drive up and see they just used the stove for a few minutes or a character said the word "fire" in a TV show they were watching.
But you have to urgently show up every time no matter what because, occasionally, the house actually is engulfed in flames and might be on the verge of igniting the whole town.