As an experienced lifter with a bodybuilding emphasis and former personal trainer I can explain this quite easily. The majority of your personal trainers are new to the field and simply have an enthusiasm for lifting. Most are still in college or just finished. They haven't really had the time to run their body through multiple pursuits such as adding muscle, losing fat, and training for brute strength.
This was me at one point. I loved lifting but my only experience was from a skinny guy trying to bulk (and I had success). I was never overweight so I didn't have the experience of losing fat. I always trained for muscle mass. Strength came with it but I never pursued strength aggressively.
So as a new/young trainer you inevitably get "mentored" by one of the experienced trainers at your gym. More often than not this guy/gal is only slightly older than you and only successful in terms of their number of clients. It doesn't actually mean they know what they're doing. I worked at a couple different gyms over the years and the most "successful" personal trainers confused their clients on the first visit and convinced them that they needed a trainer to come work out. The clients had no confidence in their own abilities so they relied on their trainers. That's great for making money but terrible as a human being. I'm a terrible personal trainer because I will teach you how to accomplish your goals on your own which puts me out of a job.
These same trainers typically jump on the bandwagon of whatever is new and popular. Example: functional training. Of course you need to stand on a Bosu ball and do single arm cable rows while balancing on one foot. It's functional. Or you could train on a stable surface and build up your strength base and train core separately and get more out of it.
So short story, long... these trainers are spouting out whatever they read on bodybuilding.com recently or what the club fitness guru told them. And by the way, the fitness guru is typically one of two people: 1) the resident meat head who has about 3 years of assisted training experience, 2) a 5'9" 150lb cardio enthusiast.
These are obviously extreme examples. Occasionally you'll find a very well educated and experienced trainer who can actually help people... but it doesn't sound like that's the case at your gym.