ELI5: Simplistically, academics don't get paid to write papers. The most important thing they get from writing papers is peer prestige. Which they need to be employed in an academic role. The academics are paid by the institutions which hire them, from research grants, and so on. The only link with financial renumeration and the papers is that the better reputation an academic has, the the better their financial situation is.

Publishers require expensive subscriptions from academic universities to be able to read their published papers.

So actually it goes the other way around. Academics have to pay to read papers, and to write papers.

Scihub does not steal a dime from any original contributor. They actually save money from them.