There is a very good reason for that; most of it is pure dross.
'Chicken and Egg' or 'Monkey and Peanuts' springs to mind. A few years ago I was asked to take some photos for a trout fishing magazine featuring some new types of lures being imported by a friend of mine. There was one of the regular contributors going to write the piece and my friend would be featuring along with his Trout Poppers. The pro' writer's regular photographer was unavailable, hence the request for me to step in.
I did the gig and must have done OK because the writer contacted me later and asked if I would consider working with him as his full time photographer. The going rate for a three page magazine article back then was £300. That was to share between the writer and photographer and had to cover travelling costs and any other expenses. OK, the day tickets and boat hire were usually free, but imagine an average day of travelling 200 - 300 miles round trip, 8 hours fishing, or in my case waiting for the action from another boat, and all this weather dependant. Working on editing, cataloguing and preparing photos is very time consuming. Also, there was no guarantee of the article being published, and if it was, usually it was the following year as magazines work seasonally and are two month's ahead of time in production.
From my days submitting photos and article to photography magazines I knew just how fickle and irrational editors can be. So, I declined the offer. There were other considerations too; I already was working full time and doing shift work, and I also couldn't stomach doing the product placement that was an obligatory part of the articles.
Even today, the going rate for a three page illustrated article in a magazine is around £400. Not many people can write and take photos well enough to be published and in the case of angling, you can't really do both most of the time so that has to be split. If magazines want the quality of content to improve they have to pay a reasonable rate AND form a working relationship with their contributors instead of treating them like s**t.