"How come volunteer fire departments exist?" is a rethorical quesiton. Whether volunteer firefighters have other jobs is beside the point. Clearly, useful tasks can be executed without a work context.

The reason you need to pay firefighters is because society is work-centric.

As a rhetorical question, I don’t think it achieves the goal you intend it to.

The reason you need to pay firefighters is because a few people might give some of their time, but a fire department needs to operate 24x7.

When you can’t find enough volunteers in your society, how do you solve the problem of buildings burning down at a time no-one chooses to volunteer?

I think people would volunteer. Main reason not many volunteer is because they are busy with work. In a workfree society people would have much more free time to spend than they have now.

I’m glad you think that. Would you volunteer? I don’t think I would. Especially late nights and weekends.

I’d rather do something more fun if I can pick and choose. Maybe race car driver? Or chocolate taster?

Yes, I would. It would be a fun way to meet new friends.