I am a chief editor of two scientific journals: Journal of Education Culture and Society (in English) , Gardens of Science and Arts (in Polish). There are many reasons for non paid review. Free reviewing is a part of the mission of the researcher. If I write a review- I can suppose that the others will do the same to me. The other reason is prestige. If you are cooperating with an interesting, prestigious journal- it is important for your scientific career.
Is it true that a lot of good researchers live an average life as compared to clinicians who have a monetary edge. Arent research posts supposed to me the most highly paid jobs.
It depends on the quality of the researcher, discipline and on the additional work. Some of the researchers, concentrated only on grants are very well paid, but it is very difficult to find many grants in... history or philosophy. In Poland in the field of education there is a lot of opportunities for grant seekers, but only if the researcher is a specialist in European Founds, like in Erasmus+ actions.
and pay to get them published in cheap or good journal and put together grants to acquire that sum. All in all, in a nut shell real science died almost 2 decades ago.