What you probably haven't been told is that the "more effort" typically means paying a rather exorbitant sum of money to the publisher to have the paper opened up.

You can certainly do that - but nobody does because for that fee you can pay two postdocs' salaries instead. Also good luck justifying that to the university bean counters - "Why do you need this $4000 fee paid when you can publish it for free?!".

The deck is intentionally stacked so high against open access publishing that it is basically unviable in the form it exists today.