What is the main reason why people wash out of Service Academies? Is it because they are too hard? Or is the because they like the civilian life better?
Can only speak for USNA but the #1 reason people don't make it through is b/c they don't really want to. Seriously.
Whether they come to this realization during plebe summer or at some other point during the first two years, they simply don't have the desire/motivation/commitment to stay. That isn't a criticism, just a fact.
Yes, quite a few leave for academic reasons. Some of those are simply "mistakes" -- USNA thought, based on their records, that they could handle the academics but it turned out that, despite their best efforts, they couldn't. That's a minority. According to USNA, most (not all) who leave for academic reasons didn't put in the effort/weren't sufficiently motivated to pass.
At USNA, it's very rare for someone to be dismissed for medical reasons; typically, they are allowed to stay but may not be able to be commissioned. It's also relatively rare for someone to be dismissed for honor now that there is honor probation. Still happens, but very rare. Some folks are kicked out for failing PT but, again, most likely an effort/motivation issue.
The current view at USNA is that, if you want to be there, you can make it through. There are a few exceptions (as noted above), but not many. Those who leave want to do something else with their lives, which is unfortunate (in that they took the place of someone who might have stayed) but understandable.