The core issue is that journals offer standard. People don't like to bother with checking every paper's reputability with a magnifying glass, and the reputation of the journal gives a shortcut around that. It's editorial work which gives the competitive edge. Now, of course scientists could run a reputable journal for free or on donations. However, once you have achieved a reputable status with your journal, it becom…
We can't even get critical infrastructure open source projects funded by donations. What makes you think people will start donating to free research journals?