The war to free science: How librarians, pirates, and funders are liberating the world’s academic research from paywalls

I think one of the answers is recognition of work and funding: more reputable journals bring better recognition and funding, so authors gravitate towards that. As proper OA journals will gain reputation, more authors will gravitate towards that.

You'd expect scientists to be a more ethically aware / idealist bunch, but there are so many that don't care at all.

I feel that the answer is probably the profit from charging authors wouldn't be as much as charging libraries but then again, these publishers already has monopoly, aren't they able to charge whatever amount they want anyway?

These costs are based on what the market will bear, they don't have any basis beyond that. Charging libraries for the subscriptions has a few advantages for the publishers: they can negotiate different fees with different institutions (which is harder to get away with on OA publication fees), they get to dip into two different budget pools (library budget as well as the actual research grants), and they get to charge institutions don't publish but still need to access research.

In short, double-dipping makes them more in total because it obfuscates the total cost, and because it makes the total addressable market larger. They've decided that authors probably will put their foot down before they can get away with squeezing an equal amount of revenue just from them.