Academics get their income from their university positions, and don't get any royalties from sales of their articles. Instead, the benefit they get from publishing is to their reputation, and for that it's better for their work to be as available as possible.

It's completely different for a writer who gets their income from sales of their work, obviously

Yep. And not that you asked, but my own opinion (not theirs) is that even writers who get income from sales will be fine either way. Reading a book for free and then buying it to support the author if you want to has been a practice for longer than the internet has existed. It's exactly how libraries have always worked!