I am a professor at Purdue University, a state school with a large engineering program, and very active ROTC, so kind of similar to the academies in some respects. (my daughter want to attend USCGA next fall).
I looked up our official data, and for the 2000 freshman class at Purdue (the last one to have 6 full years of data), 6,630 freshman showed up in the fall, 86% continued into their sophomore year (so a 14% attrition rate over the first year). Only 41% of the 2000 freshman class graduated in 4 years (and that was a record high....typically it is in the high 30's). 66% graduated after 5 years, and 72% of the incoming class graduated in 6 years.
Based on my personal experience, I suspect about 1/2 of the "dropouts" in fact left college, the other 1/2 probably transferred out to other schools (or took longer than 6 years to graduate!). Many left for the same reasons listed by others; poor time management, not ready for college, not what they expected, etc..
So the numbers at USCGA and the other academies don't really look so bad.