Scientific writers should be paid, but most of them receive grants for specific research or do their research in their function as employed researchers/lecturers at some university. Their work is then posted on some scalping journals that are charging the author a listing fee, and then charge readers an access fee, which they pocket in its entirety. Scientific authors receive ZERO compensation for their work.

Source: Used to work in academia, published a few articles myself. Best case is that your work is considered "outstanding" and the journal graciously lists it for free instead of charging you for it.