Having gone through training in PA school and in medical school, I feel that there's quite a bit of misinformation on this thread. To answer the OP's question:
1. Why do people recommend physicians assistant to premeds who don't get into medical school or to medical students who fail out of medical school?
Because they don't know what they are talking about. It is very silly to recommend PA school to someone who has failed out of medical school. Simply put, (with RARE exceptions) they will not be accepted. There are so many highly qualified applicants trying to get in to PA school as it is. Someone with a glaring red flag like failing out of medical school would have little chance of being accepted. Can it happen, sure, but it's extraordinarily rare. Sorry, that's just a cold, hard fact. Sadly, too may people have been offered this useless advice and all it does is foster false hope. As far as applying to PA school because you don't get in to medical school is concerned: Yes, plausible. So long as you have the pre-reqs, a competitive GPA, required experience and a good story as to why you want to be a PA, you have as good a shot as anyone. Just don't think that a 2.7 GPA and 40th percentile GRE is going to cut it, it won't.
2. Isn't PA school just as hard to get accepted into?
PA school is getting tougher to gain admittance to all the time. It's such a popular profession for so many people looking to operate at a pretty high level in the medical world with fewer of the demands of medical school and residency. Overall, I feel that getting admitted to MD or DO school is probably more difficult in terms of the required coursework, research and MCAT. However, PA school has very significant barriers to entry that don't lag quite as far behind as many of the advice givers on this site seem to believe, especially with respect to GPA.
3. Is PA just as difficult as medical school?
No it is not. PA school is academically rigorous, without a doubt, and excellent preparation for the creation of mid-level clinicians. There are large volumes of information that you must assimilate in PA school delivered by a curriculum with a similar layout to medical school. You are prepared well. So well that Physician Assistant is a poor descriptor for the training, I feel that Physician Associate is a better term for what you are trained to do. Medical school, however, is a whole new level of pain. I found it to be significantly more detailed, more voluminous and more demanding in terms of what you are responsible for knowing. There is a much deeper level of understanding and integration of the basic sciences with the clinical subjects you learn. Perhaps the greatest difference I appreciated was the focus on Pathology. Also, the board exams in medical school are significantly more detailed and difficult compared to the PANCE or PACKRAT.
3. I figured physician assistant schools are just filled with medical school quality students who didn't want to invest 7+ years of their lives and crazy student loans to train for a job.
You may be right here to some extent. I feel that the majority of students in PA school have the ability to do fine in medical school if that is where they ended up. At the same time, there are also PA students who are struggling in their respective programs who would be absolutely eaten alive in medical school before the end of the first semester. Overall, I think that you just adjust to whatever your environment happens to be. It's like being in a surgical rotation versus a tough outpatient clinic rotation. If you are subjected to waking up at 3 am to be in the hospital by 4:30am, work for 16 hours a day for 8 weeks, with night calls and weekends, after a while you get used to the new normal. It sucks and you know it, but you're kind of used to it and you do it (that's kind of like medical school). You also realize that your outpatient medicine rotation of 10 hour shifts, 5 days a week with 2 weekends over 4 weeks wasn't really as tough as you thought. It certainly was no cake walk at the time you were going through it, but you never fully appreciated how much worse it could be (kind of like PA school). I think this is a reasonable analogy however imprecise it may be.
In response to some other posters who were suggesting that PAs only see low acuity cases, please know that is only your experience. The fact remains that many PAs work in the ICU, as solo night house officers, in main ERs, etc, handling high acuity cases with physician oversight appropriate to their personal level of experience. Sometimes that means they're glued to the attending physician and sometimes that means they're working nearly independently. It all depends on their experience, what they know and where they work.