If, however, for-profit academic journals charge much more than it costs to produce them, I would expect to see for-profit startups competing with them.

Possible explanations for not seeing that:

• There are for-profit startups competing with the big boys, but we just don't know what they are, because of course they're smaller and less visible than the big publishers
• Startups get bought out by big publishers
• Social barriers to entry like the impact factor issue you mention
• Social barriers to entry amplified even further because startups are a bunch of entrepreneurs breaking into the field from outside, so they don't have the same connections as scientists working in the field (less of a problem for big publishers because they can buy existing publishers)