That's not at all true. From the experience of someone with a Ph.D. who is teaching at a med school (hence, working with med students all day), AND who was accepted to med school back in my day, but CHOSE a Ph.D. instead, and who has also worked with those med students who are considered the "cream of the crop" who are admitted into the M.D./Ph.D. programs, I can tell you that med students could NOT handle Ph.D. level work. Sure, a few could, but they don't have that level of interest in learning the science in depth. They are happy with a superficial knowledge that is sufficient to make a diagnosis and no more. I struggle to get them to look at the science deeper every day.
I think you've basically made up your own assumptions with NO knowledge of what work goes into any of those degrees. There is a reason that Ph.D.s are expected to teach the med students but M.D.s are not asked to teach graduate students, and that's because in any subject area, the Ph.D. knows far more than any M.D. does. The exception are the rare few who get an M.D./Ph.D., but for them, it's more a matter of survival trying to complete two degrees simultaneously.
Again, as someone who changed paths, it has nothing to do with ability to get into med school. I was accepted to med school. I changed my path for several reasons.
First, I just LOVED the level of inquiry addressed in a graduate program. Really, doing research is a passion, as is teaching a passion. Getting a Ph.D. allowed me to do both. Someone who could not get into a med school would not be the least bit successful in grad school.
Second, for what it's worth, M.D.s make a lot of mistakes. Everyone makes mistakes. But, I realized early on that I was not comfortable with the idea of my mistakes potentially killing someone. If I make a mistake in my research, I might lose time and money, but I'm not killing anyone. And I'm not arrogant enough to think I would never make a mistake in my career; that's just human nature. Much of the interesting cases we present to our students for teaching purposes are based on some fairly disastrous mistakes made by M.D.s in treating patients.
Third, if you think a TA (this is not a Ph.D. anyway, they are still students, and could be in only their first year) is not sufficiently knowledgeable in the subject yet to teach it well, try asking a med student to teach it to you.

Actually, ask an M.D. to teach it too you! You'd appreciate your TAs better. I've spent a good deal of time correcting residents (they have M.D.s) and fellows (subspecialty training after they are licensed to practice!) on some fairly basic biological principals.
Fourth, the part I enjoy most out of what I do is the teaching. This was part of the basis of my decision to get a Ph.D. instead of an M.D. As an M.D., I could impact the lives of my patients by doing the best I possibly could do to treat them and ensure they had good quality care (...or spend the rest of my life filling out insurance forms, since that's really what M.D.s spend inordinate amounts of time doing), or I could impact all the patients of generations of M.D.s as each of them comes through my classroom and I have an opportunity to make every one of them a better physician.