Most of the curators on SubmitHub are doing so because they love finding music, not because they get paid for it. 95%+ do it in their spare time as a hobby/passion project.
The benefits of doing standard reviews include:
1) Helping out indie artists who can't afford premium credits;
2) Discovering great new music that not many other people will have heard;
3) They're way easier than premium submissions — no feedback required. You get to sit back, listen, and make a decision.