It's probably not that clear cut. Maybe they have a full time job. Maybe their employer has An Opinion regarding side gigs. Maybe they live somewhere where a side gig comes with a lot of paperwork/taxes so it isn't worth the hassle.
There's probably some price where it's worth the hassle. I feel like every OSS project starter should have at least a firm notion of where that line is before they publish. On one hand, maybe you'll get it, but mostly it helps clarify what your time is worth so that you don't feel guilty about rejecting requests on your time that don't meet your own standards for value.
But they could! It's as easy as saying "I can consult on $topic, please contact me to discuss details."
Then you charge money and do the work.