Most of this work is done by academics who are either not paid or paid a nominal sum. Yes, running a journal is a lot of work, but most of the people who do the actual work do not reap the rewards.
Academics usually do the peer reviewing and sometimes the editing (which is almost certainly a waste of their talents as a good editor is cheaper and more efficient at editing than some Biology PhD guy).
Most of the more prestigious journals (e.g., the ones people are paying for) are supported by publishing house staff doing the sort of stuff I mentioned -- or, more likely, managing the vendors in India who are doing the stuff I mentioned.
Source: I worked in scientific publishing for many years. Happy to be out of it...
Since most subscriptions are sold in "big deal" packages, where few of the most prestigious journals get bundled with a lot of the less prestigious ones to get libraries to purchase more of the same publisher, it's not just those ones that people are paying for...