There are many reasons. I know, I know, I know is a common answer trainers give to something they repeatedly make a mistake on and one of my pet peeves. So fucking irritating. My common answer to the I know remark is, then if you know, why the hell do you keep making the same mistake REPEATEDLY schmuck?
I think the most legit excuse for a trainer being a danger, is due to their age, but this hall pass has a very short expiration date, because I see too many who are older that make young and dumb mistakes. Age is certainly not the only factor for the creation of (danger trainers).
Too many of these danger trainers are not in it whole heartedly. In Hollywood la la land many act as personal trainers in between auditions for a movie part. Based on their personal training skills, I'd say they're shitty actors.
Then you have those trainers who are part time trainers, or doing it as a second job. This is all fine and dandy, but the real difference here is personal training is not like flipping burgers. You can actually fuck people up if you don't know what you're doing. I know you gotta start somewhere, but do it on yourself or one of your family members for fuck sakes.
Finally, there are those who don't take it seriously, as in realize that this is a real job. It just happens to be a great one, where you actually make differences in people's lives, as in life altering changes. The beautiful thing here is this job gives the gift that never stops giving. It's a lifestyle friendly job. Imagine having the kind of flexibility where you positively impact lives, with the ability to live your life on your terms.