Possibly it's because people boot them around more nowadays, also more fungicide, pesticide and everythingicide is used on farms. As Thunderbird+ has already explained, the toadstools are all a part of the same thing and are linked together by mycelium. Thus you can tell the approximate age of it by the radius of the ring, as each year it expands (only the most distant points of the fungi produce the spores, these are therefore the newest bits). Obviously then new ones are just a small cluster of toadstools when they spore. It's only the older ones that make wide rings (ie. the ones which haven't been hoofed around or subjected to fungicides, either applied directly or via leeching from nearby farmland). In other words, perhaps most of the ones you see today are all younger as they haven't had the chance to grow much before they get killed.
It's just a theory though so correct me if I'm wrong.