You can argue whether we're volunteers or not. Staff are paid every month, but we're not technically employees.
On most forums I know, moderators aren't paid at all. And most forums have much less spam too. It feels like a fight that was lost the moment spammers got away with earning money from their spam.
It's a vicious circle: if the spam would be handled fast enough, they wouldn't earn from it, and spam wouldn't be worth it. At the moment there's too much spam to handle, they get away with it, and earn money.
I've been a mod of several forums before and you're right that they're normally just volunteer positions but this forum is pretty unique really with the amount of money that is able to be earned here which just exacerbates the problems as the forum continues to grow at an exponential rate. As with most forums, they tend to go through staff pretty quickly as it's largely a thankless task and this would be infinitely worse if there was no mod payments as people would just give up because it wouldn't be worth the stress or hassle. Many staff have already walked away or got burnt out over the years (BadBear, tysat, miningbuddy just to name a few), and there's quite a few more that seem to get less and less active with each passing month (grue and some of the local mods etc), and the more and more users that sign up here the bigger the workload gets piled on existing staff, so that's why the workload should be distributed a bit or more staff added to meet demand.