It is required for schools to have attrition rates posted on their website. However, they tend to fall off sometimes at opportune times, such as during interviews. When I finally found my school's attrition rates, I realized they would lose 2 or 3 students per year, out of 5 or 6 - almost 50%. I realized this only a few weeks in, and I realized I was going to be the first one to step out in my year. A year later, 3 were gone of the 5 that were admitted. One had done a PhD for her parents when she actually wanted to go to law school. Another realized she wanted to have an actual life sometime in the next five years, get married, have kids.
There are countless reasons that people leave. However, the difference between various programs' rates demonstrates the severity of issues with programs specifically. Programs with set times, such as law schools and medical schools, have attrition rates often in the single digits. Social sciences can be as high as 40% while humanities often hovers around 29%. Further, different schools can have very different rates.
I suggest that anyone investing their time, money, and life into a program do the research. You have a right to know what you are getting into.