Trainers vs "Trainers"
Just because you go to a gym doesn't qualify you to train someone.
And to be clear, this does include those trainers who are certified. Certification means nothing.
There's a lot to know and learn. Most of which is only going to be learned through experience and years of it. I've been lifting for about a decade now and I'm still learning new things that I like better and have seen produce better results. Even the basics take time to nail. I still am on a never ending quest to perfect my squats, deads, and bench press to find what works best for my body. People underestimate the value of mental strength and willpower. They underestimate the value of being able to actually feel the muscle or muscle group working that you are focused on.
The truth in health and fitness is that it's more like an art than a science. Science is definitely part of it, but no one person reacts the same, so you must have the experience to think outside the box and "bend" the standards to get results for people and again, that takes time. You will not learn this from a book, a class or a study guide.
Then, on top of it all, there's nutrition...
As they say:
"Just because you live in a house doesn't mean you're qualified to go build one."