I'd bet 25 mile weeks are better for one's health than 70-120. They're likely not trying to get to the absolute limit their body can reach or even hit qualification standards for large and popular races like Boston or Berlin marathons. Some probably do, but those're just stupid. One needs at least 40-50 mpw to get decent at a marathon, most people probably even more.
But for those who're not trying to get 'decent' running 25 mpw in 3-4 runs per week is a great healthy hobby, and it's not like they're coming into marathon completely unprepared, they're just not pacing it correctly for their ability.
As for why the distance, well here's my answer. Finishing under cutoff time is hard. Vast majority of people won't be able to finish a marathon on no training at all. Anyone who can walk can 'run-walk' finish a 5K, most people would be able to do this with a 10K as well. As for the half I might be biased because when I took up running as an adult, I entered a local one and finished it under 2 hours having ran about 30 miles over the two months before the race(other training was weight lifiting and ~40 min walking commute to office one way). Either way marathon is 'locked' from those without at least some tiny dedication.
So as the challenge made it first choice of distance for recreational runners of 30-40-50 years ago, marathons became more massive, thus attracting more runners and spectators and this is like self-supporting now. Every major city has a marathon and people often plan their holidays like 'go to new york, eat some local food, visit MOMA, run the race'. There are a few races with other distances which attract a lot of runners and spectators, too. Like Lindingoloppet in Sweden which is a trail 30K