I ran for a D1 program in the Pac 10 (later Pac 12) from 2008-2012. Our attrition rate was really high. Off the top of my head, here is some data on our team starting with the class a year above me listed by the year that the athletes began school (male distance runners):
2007- We started with 8 recruits, 2 more walked on. 2 people finished all four years on the team and one of them was a walk on. Attrition rate 8/10- 80%
2008-We started with 8 recruits and 1 walked on, at the end of four years two people were left and one was the walk on. 7/9- 78%
2009- We started with 8 recruits and 1 walked on, at the end of four years 5 were still on the team. 4/9- 44%
So in those three recruit classes we had 19/28 (68%) people fail to make it through 4 years on the team. There were many different reasons for athletes leaving. We had a high incidence of injury. A few people quit to focus more completely on academics. More left to join fraternities or simply got caught up in the more "typical" college lifestyle while they were injured and never made it back. I was one of the kids who didn't make it all four years. I ran for 3 years (6 seasons counting xc and track). I was injured for 4 out of those 6 seasons. I simply got sick and tired of dedicating my life to something and still failing to improve, or even make it to the starting line to see if I had improved. It was all fun and games in high school when I ran PRs every season and was competing for the win in the majority of my races. It was hard to justify the sacrifices it takes to run at a high level when I hadn't even been healthy enough to race during the season in years. College running is a great experience for those who are healthy and running well, for anyone else it can be incredibly depressing.