I think it's true that most fights end up on the ground.
I think it has something to do with wrestling. I remember back in middle school, everyone was required to do a quarter of wrestling in P.E., which was the only form of acceptable fighting there was in school, and for most people, that quarter would be the extent of their combative training for life there after. Also, most of the kids these days watch the WCW, WWF etc. and mimic the moves of there idols. So, I believe that wrestling on the ground is a more natural thing to do when you don't know how to skillfully fight standing up. And you have to admit that probably 95% of people in today's societies have never been trained to fight. Also, "slamming" people on the ground seems to be a popular thing to do. And people tend to do what's more popular.
I've often wondered what would happen if the ref. never seperated boxers after they've gotten into a clinch. I'd like to see a tired boxer try and clinch a grappler.
As far as fights in prison etc. As a youngster, I spent almost 4 years in the California Youth Authority (prison for people up to age 25 (I was 16). All of the fights I saw ended up on the ground. Every single one of them. These guys would start by flailing their arms towards each other, then get into a short clinch, then one or both would attempt to slam the other to the ground either trying to slam them against an object or bash their heads against the ground. Other times, it would be a sneak attack from behind with a sock full of canned goods or batteries. Someone would get knocked to the ground and then the fight would continue there.
I believe that everyone that desires to be able to protect themselves should include ground fighting as a mandatory ingredient in their skills. At least some basic reversals, submissions, and positioning. Because, what if you miss? What if you make a mistake and fall to the ground? You need to know how to safely and effectively get back up.
I also suggest that people who desire to train in Aikido (for self-defense), first train in other forms of MA. A ground fighting art, and a striking art. Even if just for six months or how ever long it takes you to get the basics down and to learn positioning and stances. Just having that experience will give you such an advantage in defending against those types of attackers, and help you understand the purpose of Aikido techniques. I've trained in BJJ (2years), Tai-Chi Chuan(6months), and a family style of mixed Chinese martial arts(6months). Even though these were short periods of time, I trained 8 hours a day 5 days per week(for the chinese arts), and later BJJ several hours per day 5 days per week w/alot of private instruction.
BJJ is awesome 1 on 1, but is almost worthless against multiple attackers. Although, one time I was attacked by two people IRL and it ended up on the ground and I was able to use my ground positioning skills from BJJ to keep these guys at bay and to prevent them from seriously harming me. It's worth it. It saved my life.
Oh yeah, I almost forgot. The Aikido instructor at my BJJ school used to attend some of our classes, and he used to get his butt whooped by the BJJ instructor. But then again, I never witnessed the BJJ instructor try and attack the Aikido instructor from a standing position